Hello Annie, and welcome to Melnet! I think everyone has trouble with the basses at first, it's just one of those "rub your stomach and pat your head" things that is hard to untangle until you practice it a bit.
You could start with just playing a simple oom-pah oom-pah rhythm on the left hand and once that is steady just start adding some right hand notes. Just play some notes in rhythm with the basses, and then once you've got that, try playing up and down the scale while maintaining the rhythm.
When you are first learning, it is usually OK to just stay on the pair of buttons with the root and the 5th bass and chords. (So in the key of G, the lower left pair of buttons.) Most of the time the chords will fit the melody, more or less.
Are you learning from a book? I used Dave Mallinson's "D/G Melodeon - Absolute Beginners" and I thought he does a good job easing the new player into using the basses.