It’s best to pre-drill all holes slightly undersize at first, then drill the hole to full size. This tends to make more round holes, as the final sized bit is not removing as much metal. For thin sheets of aluminum, you want a fast air drill. For steel, you need a slower turning drill, with lots of torque, so an electric drill is better. A drill press facilitates bit angular alignment, so use a drill press whenever possible. When drilling holes larger than 3/16” or so in thin sheets, consider using a step drill. These are more likely to drill a round hole than a traditional twist drill, and if you aren’t using the largest size step, you can use the same bit to debur the back of the hole.