It's odd to sew a straight line at first. When I first sewed on a machine, I made sure my fabric was cut straight. So when I sewed, I made sure the same amount of room was on the outside of the stitch. Then it's just one of those feel things when you're sewing.

A hint from me? Go fast. When you sew slowly, there's more room for error. It may also help to chalk where you want to sew. Obviously, chalk the inside where you sew. For example, if you're sewing down the leg of a pair of jeans, you don't want it exactly vertical and perpendicular with the ground, you just want the stitches to be straight.. so chalk the measurements and such. I hope that makes sense.