add and remove app

Open a Terminal from the menu Applications → Accessories → Terminal and type:
(if the system ask you a password give your user password, you will not see nothing when you type it, then press enter)

sudo apt-get update
sudo apt-get upgrade
sudo apt-get dist-upgrade

Hope this helps

i did it n i got this message:
  404 Not Found
Err http://ppa.launchpad.net intrepid/main Sources
  404 Not Found
W: Failed to fetch http://ppa.launchpad.net/gscrot/ubuntu/dists/intrepid/main/binary-i386/Packages.gz 404 Not Found

W: Failed to fetch http://ppa.launchpad.net/gscrot/ubuntu/dists/intrepid/main/source/Sources.gz 404 Not Found

E: Some index files failed to download, they have been ignored, or old ones used instead.

Using the menu System → Administration → Software sources click on to the "Thirdy-Party Software" tab
and please un check the
http://ppa.launchpad.net intrepid main (Source Code)
and the other ppa rows with "gscrot" in it...

Then retry...

Ok, so firstly what ubutnu version are you on (7.04, 8.04, 8.10 etc.)? Is it intrepid (8.10) as your error message suggests?
Secondly, I presume you were trying this when you were connected to the internet (without firewall)?
Thirdly, can you try to launch adept from the terminal ("sudo adept" or "sudo adept-manager" on older versions)? If that's not the case you can try to install it (sudo apt-get install adept).
Finally, the error message that you get relates to non-standard ubuntu repositories. Can you post a copy of your /etc/apt/sources.list file (use the command "more /etc/apt/sources.list" and paste the results - the space bar scrolls to the next page or open it in a text editor)?

Good morning and thank you Marcobra and Jetbundle for all the help:
I am running 8.10 intrepid, and i tried what Marcobra told me, uncheck http://ppa.launchpad.net, and 1 more i had. but with not results i got the same message :There is no matching application available. I am using my own internet without a firewall. Mr jetbundle i ran: "more /etc/apt/sources.list" like u told me and this is what i got;

alfavasquez@alfavasquez-laptop:~$ more /etc/apt/sources.list
# deb cdrom:[Ubuntu 8.10 _Intrepid Ibex_ - Release i386 (20081029.5)]/ intrepid
main restricted
# See http://help.ubuntu.com/community/UpgradeNotes for how to upgrade to
# newer versions of the distribution.

deb http://us.archive.ubuntu.com/ubuntu/ intrepid main restricted
deb-src http://us.archive.ubuntu.com/ubuntu/ intrepid main restricted

## Major bug fix updates produced after the final release of the
## distribution.
deb http://us.archive.ubuntu.com/ubuntu/ intrepid-updates main restricted
deb-src http://us.archive.ubuntu.com/ubuntu/ intrepid-updates main restricted

## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu
## team. Also, please note that software in universe WILL NOT receive any
## review or updates from the Ubuntu security team.
deb http://us.archive.ubuntu.com/ubuntu/ intrepid universe
deb-src http://us.archive.ubuntu.com/ubuntu/ intrepid universe
deb http://us.archive.ubuntu.com/ubuntu/ intrepid-updates universe
deb-src http://us.archive.ubuntu.com/ubuntu/ intrepid-updates universe

## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu
## team, and may not be under a free licence. Please satisfy yourself as to
## your rights to use the software. Also, please note that software in
## multiverse WILL NOT receive any review or updates from the Ubuntu
## security team.
deb http://us.archive.ubuntu.com/ubuntu/ intrepid multiverse
deb-src http://us.archive.ubuntu.com/ubuntu/ intrepid multiverse
deb http://us.archive.ubuntu.com/ubuntu/ intrepid-updates multiverse
deb-src http://us.archive.ubuntu.com/ubuntu/ intrepid-updates multiverse

## Uncomment the following two lines to add software from the 'backports'
## repository.
## N.B. software from this repository may not have been tested as
## extensively as that contained in the main release, although it includes
## newer versions of some applications which may provide useful features.
## Also, please note that software in backports WILL NOT receive any review
## or updates from the Ubuntu security team.
deb http://us.archive.ubuntu.com/ubuntu/ intrepid-backports main restricted univ
erse multiverse
deb-src http://us.archive.ubuntu.com/ubuntu/ intrepid-backports main restricted
universe multiverse

## Uncomment the following two lines to add software from Canonical's
## 'partner' repository. This software is not part of Ubuntu, but is
## offered by Canonical and the respective vendors as a service to Ubuntu
## users.
deb http://archive.canonical.com/ubuntu intrepid partner
deb-src http://archive.canonical.com/ubuntu intrepid partner

deb http://security.ubuntu.com/ubuntu intrepid-security main restricted
deb-src http://security.ubuntu.com/ubuntu intrepid-security main restricted
deb http://security.ubuntu.com/ubuntu intrepid-security universe
deb-src http://security.ubuntu.com/ubuntu intrepid-security universe
deb http://security.ubuntu.com/ubuntu intrepid-security multiverse
deb-src http://security.ubuntu.com/ubuntu intrepid-security multiverse
# deb http://ppa.launchpad.net/gscrot/ubuntu intrepid main
# deb-src http://ppa.launchpad.net/gscrot/ubuntu intrepid main

So it seems that your sources.list is allright now.
Does "sudo apt-get update" still give you error messages like "heeeelp, can't find the repository"?
And can you try to install any random package using the command line "sudo apt-get install [package name]" (just take any package that you know is available but you don't have installed)?

Good morning Mr Jet i did sudo apt-get update n this is what i get:
alfavasquez@alfavasquez-laptop:~$ sudo apt-get update
Hit http://security.ubuntu.com intrepid-security Release.gpg
Ign http://security.ubuntu.com intrepid-security/main Translation-en_US
Hit http://archive.canonical.com intrepid Release.gpg
Ign http://archive.canonical.com intrepid/partner Translation-en_US
Ign http://security.ubuntu.com intrepid-security/restricted Translation-en_US
Ign http://security.ubuntu.com intrepid-security/universe Translation-en_US
Ign http://security.ubuntu.com intrepid-security/multiverse Translation-en_US
Hit http://security.ubuntu.com intrepid-security Release
Hit http://archive.canonical.com intrepid Release
Hit http://archive.canonical.com intrepid/partner Packages
Hit http://security.ubuntu.com intrepid-security/main Packages
Hit http://us.archive.ubuntu.com intrepid Release.gpg
Ign http://us.archive.ubuntu.com intrepid/main Translation-en_US
Hit http://packages.medibuntu.org intrepid Release.gpg
Ign http://packages.medibuntu.org intrepid/free Translation-en_US
Hit http://archive.canonical.com intrepid/partner Sources
Hit http://security.ubuntu.com intrepid-security/restricted Packages
Hit http://security.ubuntu.com intrepid-security/main Sources
Hit http://security.ubuntu.com intrepid-security/restricted Sources
Hit http://security.ubuntu.com intrepid-security/universe Packages
Ign http://us.archive.ubuntu.com intrepid/restricted Translation-en_US
Ign http://us.archive.ubuntu.com intrepid/universe Translation-en_US
Ign http://us.archive.ubuntu.com intrepid/multiverse Translation-en_US
Hit http://us.archive.ubuntu.com intrepid-updates Release.gpg
Ign http://us.archive.ubuntu.com intrepid-updates/main Translation-en_US
Ign http://us.archive.ubuntu.com intrepid-updates/restricted Translation-en_US
Ign http://packages.medibuntu.org intrepid/non-free Translation-en_US
Hit http://packages.medibuntu.org intrepid Release
Hit http://security.ubuntu.com intrepid-security/universe Sources
Hit http://security.ubuntu.com intrepid-security/multiverse Packages
Hit http://security.ubuntu.com intrepid-security/multiverse Sources
Ign http://us.archive.ubuntu.com intrepid-updates/universe Translation-en_US
Ign http://us.archive.ubuntu.com intrepid-updates/multiverse Translation-en_US
Get:1 http://us.archive.ubuntu.com intrepid-backports Release.gpg [189B]
Ign http://us.archive.ubuntu.com intrepid-backports/main Translation-en_US
Ign http://us.archive.ubuntu.com intrepid-backports/restricted Translation-en_US
Ign http://us.archive.ubuntu.com intrepid-backports/universe Translation-en_US
Ign http://us.archive.ubuntu.com intrepid-backports/multiverse Translation-en_US
Hit http://us.archive.ubuntu.com intrepid Release
Hit http://us.archive.ubuntu.com intrepid-updates Release
Get:2 http://us.archive.ubuntu.com intrepid-backports Release [44.0kB]
Hit http://packages.medibuntu.org intrepid/free Packages
Hit http://packages.medibuntu.org intrepid/non-free Packages
Hit http://packages.medibuntu.org intrepid/free Sources
Hit http://packages.medibuntu.org intrepid/non-free Sources
Hit http://us.archive.ubuntu.com intrepid/main Packages
Hit http://us.archive.ubuntu.com intrepid/restricted Packages
Hit http://us.archive.ubuntu.com intrepid/main Sources
Hit http://us.archive.ubuntu.com intrepid/restricted Sources
Hit http://us.archive.ubuntu.com intrepid/universe Packages
Hit http://us.archive.ubuntu.com intrepid/universe Sources
Hit http://us.archive.ubuntu.com intrepid/multiverse Packages
Hit http://us.archive.ubuntu.com intrepid/multiverse Sources
Hit http://us.archive.ubuntu.com intrepid-updates/main Packages
Hit http://us.archive.ubuntu.com intrepid-updates/restricted Packages
Hit http://us.archive.ubuntu.com intrepid-updates/main Sources
Hit http://us.archive.ubuntu.com intrepid-updates/restricted Sources
Hit http://us.archive.ubuntu.com intrepid-updates/universe Packages
Hit http://us.archive.ubuntu.com intrepid-updates/universe Sources
Hit http://us.archive.ubuntu.com intrepid-updates/multiverse Packages
Hit http://us.archive.ubuntu.com intrepid-updates/multiverse Sources
Get:3 http://us.archive.ubuntu.com intrepid-backports/main Packages [70.8kB]
Get:4 http://us.archive.ubuntu.com intrepid-backports/restricted Packages [14B]
Get:5 http://us.archive.ubuntu.com intrepid-backports/universe Packages [29.4kB]
Get:6 http://us.archive.ubuntu.com intrepid-backports/multiverse Packages [14B]
Get:7 http://us.archive.ubuntu.com intrepid-backports/main Sources [13.8kB]
Get:8 http://us.archive.ubuntu.com intrepid-backports/restricted Sources [14B]
Get:9 http://us.archive.ubuntu.com intrepid-backports/universe Sources [10.1kB]
Get:10 http://us.archive.ubuntu.com intrepid-backports/multiverse Sources [14B]
Fetched 168kB in 6s (25.3kB/s)
Reading package lists... Done
alfavasquez@alfavasquez-laptop:~$
And i go to add n remove and i get this:
There is no matching application available.
Then i tried to install sudo apt-get install cinelerra and this is what i get:
alfavasquez@alfavasquez-laptop:~$ sudo apt-get install cinelerra
Reading package lists... Done
Building dependency tree
Reading state information... Done
E: Couldn't find package cinelerra
alfavasquez@alfavasquez-laptop:~$

So your apt-get update output looks ok.
Cinelerra is not in the ubuntu repositories, so it's no surprise that it cant find it.

For example
sudo apt-get install gnome-chess
should either install the package or tell you that it's already installed and at the newest version