This can seem hard if you've never used a sewing machine before, but fear not it's actually quite simple (although you'll definitely need some practice). To stitch a straight line, place the edge of your fabric along one of the lines* on the needle plate, and keep that alignment while you stitch. *Most sewing machines have these lines. If not, you can "draw" one by sticking a bit of tape on the needle plate. Go slowly at first then increase your speed as you gain confidence. Always focus on the alignment, not the needle or the stitching: they move too fast for your eyes to follow plus it prevents you from seeing the bigger picture. To make a smooth curve, stitch slowly while rotating the fabric using both hands. Do this in a steady, continuous movement if you can, and if you can't (or if you need to make a sharp turn) always stop with the needle down into the fabric. This allows you to pivot the fabric around the last stitch ensuring a neat curve.