Well, if you are learning to use the rotary cutter, just go slow and keep your blade next to the ruler. You might try cutting pieces of fabric that are no longer than the ruler as opposed to cutting a 44" or 36" piece, that way you will feel more control.
If you find that you go off to the side, just line up the ruler with the part of the good edge you cut and straighten out your cut on the part that went off. You'll get the hang of it.