Here are a few tips that might help sewing a straight seam.
Slow down--it's not a race. Slower means more accurate seams.
Watch the edge of the fabric in relationship to the edge of the fabric. Do not watch the needle, which I used to do, it takes care of itself going up and down in the same spot.
Best of luck, I used to sew wobbly seams also and these two tips helped me.
Sandy