if you want to search for packages, do: sudo apt-cache search whatever-you're-looking-for
If you want to browse, install synaptic (sudo apt-get install synaptic) - run it and you'll get a great GUI frontend for apt which displays all the packages available.
Remember that you should always refresh your repository list before you install (sudo apt-get update)
Also remember that you can hit tab midways through typing a string. If nothing happens, hit it twice and you'll see all the possible packages that begins with whatever you typed.
Good luck and have fun!