how do I stop a unwanted site from getting on my pc

I am using Ubuntu 12.04 and Firefox as my browser

So do you want to block the system from being able to access the site, or do you want to block the incoming traffic if it isn't requested?

Open the file

gksudo gedit /etc/hosts

and add one line in this format

127.0.0.1 <the adderss of the site>

save the document and you are done.
This is the safest way to block the access to an Unwanted site and NONE but only you (the system admin) can change that , because as you saw , this file (/etc/hosts/) wants root access to modify it.

Thanks

Its not clear what you want to do here.

If you want to stop other people using your computer to get to some website you don't like then edit /etc/hosts as suggested by Nick,

If you want to stop an an external site from accessing your computer then by default. It should not be able to. To access your computer, your computer must be listening for connections and the default install doesn't. If something is accessing your computer we need more information

1. What version of Ubuntu are you using
2. What is being accessed
3. What site is doing the access.

I'm sure we can help further but need more details.