If you’re having trouble keeping your seams straight, you probably just need some more practice. Go slow- don’t put the pedal to the metal just yet. Some machines are really fast, so you’ll need to practice keeping steady pressure on the pedal at a speed you can keep up with. It’s really a lot like driving.
You can also get a handy little tool called a Seam Guide. It’s a little magnetic doo-dah that sticks to the plate of your sewing machine. You adjust it to fit your seam allowance, and then while you’re sewing, line it up with the edge of your fabric.
Seam guides should be readily available at your local craft/sewing supply store like Joann Fabrics.