Heh funny i developed the exact same interest not too long ago.
A lot of great songs can be learned right on good ol youtube actually, just search for the name and add "tutorial" or "synthesia". Or alternatively just DL synthesia. It's a little like guitar hero as it shows you what keys to hit. The free version has loads of classics and you can add any song as long as it's a .midi. If you have a keyboard you can also get the full version (25 bucks tho) and than it will stop until you hit the right key wich makes it much easier to learn.
As for songs, idunno if you like tf2 but I liked the spy's theme and it turned out it's not that hard: http://www.youtube.com/watch?v=D9u52bZVBdw
Also it turns out a lot of great techno songs are pretty easy too. Hell insomnia can be played on a cellphone if it has at least 4 different beepsounds. But again you'll need a keyboard with lots of effects if you want it to sound exactly like the real song:
http://www.youtube.com/watch?v=eU-dZxa6oA8&feature=related
Footnote:
MissPixxie said:
Been looking ages for that one cuz i didnt know it's name! Thanks!