First of all, make sure that you have included sources repositories in your /etc/apt/sources.list file. That is, a line which starts with deb-src should be there, like this one…
Note: You have to be root in order to install software with apt-get. Type su in a shell and enter your password. For security reasons (sometimes you will need to copy/paste a portion to an IRC channel or Pastebin in order to troubleshoot something), the password is hidden when you type it, but it goes in. Press Enter when you're done.