You can clean out that cache with:

apt-get clean
apt-get autoclean

You will have to do that as root, so install the gainroot package. Then go to the terminal and type "sudo gainroot" to become root. Then do the above commands.

If you don't understand what that stuff means yet, DON'T DO IT. Learn some basic linux first in a safer environment. I suggest installing virtualbox on your computer (or vmware server, if you are feeling braver), then installing Ubuntu in a virtual machine (you can choose another distro if you want, but Ubuntu uses .deb packages and uses apt-get, so it is more similar to your phone than some other good distros). You can do whatever you want within the virtual machine without worrying about messing anything else up.

If you just want a guide to apt-get, read this.

Happy linuxing...