Generally, paper aeroplanes do not fly very well on their first flight. Four of the most common problems that people encounter are: the plane nosediving into the ground; the plane stalling (pitching up rather than flying straight); the plane turning to one side; and the plane rolling as it flies. Fortunately, there are a number of ways to fix these problems.
I have set out the simplest ways on this page; however, if you want to go into some more complex aerodynamics, there are a number of detailed posts on my blog.
1) Nosediving (pitching downwards)
2) Stalling (pitching upwards)
3) Yawing (turning to one side)
Here the plane turns (yaws) to one side rather than flying straight. This can be rectified by tweaking either the back of the fins (or the fuselage if the plane has no fins) like a rudder in the opposite direction to the yaw. For example, if the plane yaws left, tweak the very back of the fins very slightly to the right.
4) Rolling
If the plane rolls as it flies, it usually lacks what is called 'lateral stability'. I cover the fixes to this problem in detail in a few blog posts here and here. However, the simplest fix is to make sure the wings are placed at a dihedral angle. Here's the relevant diagram from the first blog post.
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Once you've calibrated your paper aeroplane, it should fly straight and flat. Happiness and profit shall ensue.
Once you've calibrated your paper aeroplane, it should fly straight and flat. Happiness and profit shall ensue.