I use a program called Guitar Pro, which (as the name explains) is mainly a program for guitar players. But if a song has a keyboard/Piano, it's usually there as well. Then I just go to
www.ultimate-guitar.com and download some tabs.
The good sides about that program is that it shows me the keyboard, where I can see which notes are being played, so I can learn a song without being able to read the sheets.
The downsides are, that you probably won't learn to read sheets, as the other way is much easier

, and you will probably not be able to find songs that does not contain a guitar, like Beethoven or similar.