Straight lines can be really hard. Use a 1/4" foot and put a marker (ie a pad of sticky notes right on the 1/4" mark on your sewing machine. Do not go over the pad or vear to the left under your 1/4" presser foot. When you press (not iron) make sure there are no folds (no matter how small at the seam line.
Following the above and lots and lots of practice and you will be sewing straighter than an arrow - good luck :idea: :idea: