Go into practice mode. Pick a song, or even just a part of a song. Let it play through hitting only the pedal. Repeat until you can hit every pedal note in the section/song. Now, try to add the red drum hits using your left hand, and again, practice until you're doing alright with it, then try to get the right one in. If you're having problems with any of the beats, just go into practice, and break it into as simple sections as you can.
Practice, practice, practice. I was just as bad as you when I was starting, now I can play most songs (all in beatles, most in RB2) on expert.
sig will not change untill the L-shaped block of tetris & the assorted motor vehicles of frogger are in the summercontest