I'll add to the last comment... vocals can be a lot easier. A lot of people seem to be afraid to even try vocals. (I actually used to sing semi-professionally, so I'm ok with it, but I know a lot of my friends are afraid to even try.) Some pointers:
The game can't tell if you're singing the words or not, just the pitch of your voice. Even if you're not used to singing, you can practice moving your voice to follow the pitch without the lyrics.
Also, don't sing too loudly; that will push off your pitch. Instead, sing right into the microphone at a normal level. (Don't do it too quietly, either. The game can't pick it up.)
Like the last comment said, each person will have difficulties with different songs. The problem is when a song is over your "break." In those cases, you can switch octaves in the middle of the song, singing higher or lower depending on the song. Falsetto works too, and I found myself switching to falsetto during some songs because the whole song was right on my break. The game doesn't care, again it's all about pitch.
Good luck with it!