When you begin folding choose paper that is light to medium in thickness. Heavier paper works well for some airplanes but is harder for a beginner to fold. Make creases that are accurate and tight using your fingernail or a ruler. Even the first fold must be exactly right for your craft to fly well.
Most airplanes in this program need letter size paper, although legal, A4, or any rectangular paper will work. Some need square paper and cannot be folded successfully without it. An easy way to make square paper is to fold the top edge of a sheet so it lines up with the left edge. If the blue strip along the bottom is cut off what’s left is perfectly square.