How to creat an internet connection in ubuntu?

To change your MAC address you can run:

gksudo gedit /etc/network/interfaces

and add these lines:

auto eth0
iface eth0 inet static
address 192.168.0.100
netmask 255.255.255.0
gateway 192.168.0.1
   hwaddress ether 01:02:03:04:05:06

obviosly change 01:02:03:04:05:06 to whatever you need as well as the IP adresses.

as you are using static IP you will also need to run:

gksudo gedit /etc/resolv.conf

and add the addresses of your DNS servers. You can also use public DNS servers if you wish if you add these lines to that file:

nameserver 4.2.2.1
nameserver 199.45.32.43
nameserver 64.81.127.2
nameserver 204.117.214.10

And name resolution will take place. Oviously if you know your own DNS severs you should put those in above the ones I named there.

Not sure about the username and password bit. If you use a usb modem with a username / password then this may help:

https://wiki.ubuntu.com/UsbAdslModem

Click on the top panel on System-->Preferences-->Network connections. Then click under Wired on the "Auto eth0" (this is your card) and then on Edit. Give your user password for the system. A new window appears. Fill your MAC address under the first tab which should be "Wired". Under the IPv$ Settings fill the rest needed clicking on the empty spaces. Then click aply and if everything is alright in a few seconds the connection should be established.There have to be a message if connection has failed or not.
The same could be done by right-clicking on the connection icon in the right part of top panel and chosing Edit Connevtions... .

I managed to insert necessary address using the Network Manager applet form the top panel. But I had to run

 sudo pppoeconf

to establish the connection.But a problem occurred.There was a sudden failure of power and don't have an UPS.
After starting Ubuntu it was unable to connect. The name of wired connection "Auto eth0" changed to

 ifupdown (etho0)

and i cannot edit it as it is showing read/write problem.
Can you help me?

Another thing. The Network Manager Applet from the top panel just went away from the panel a day ago. I couldn't bring it back. Although System-->Preferences-->Network connections looks like the applet it doesn't work the same way as the applet of the panel. I had to reinstall ubuntu to bring it back.

Is there any way to bring it back if it is gone?

Now I'm launching from your first task:
That was: I want to use the connection in ubuntu 8.10

Please just reread your question and the answers and think: please give us of more accurate question for the reason of one can give you the proper answer.

You know - I suppose the half of the righteous answer is in the properly asked question.