When a plane has been constructed or assembled, it needs to be balanced before you fly it. It should not be nose or tail heavy. With the plane fully assembled but with no fuel in the tank, pick the plane up by the wing tips and ensure it does not tip forward or backwards.
Many planes identify their center of gravity position. This is the position on the wings where the balance point should be measured from. If the balance point is not identified in the build manual or on the plan sheet, a general rule-of-thumb is to balance the plane 25% of the "mean aerodynamic chord" (the average distance from the leading edge to the trailing edge - usually the thickest part of the wing).
If the plane is not balanced properly, there are a number of options. Firstly, if the location of the wing can be moved forward or backwards, that is usually a good idea. If the plane tips forward, move the wing forward. If the plane tips towards the tail, move the wing towards the tail. Basically move the wing in the heavy direction.
For planes which use wooden dowels placed through the fuselage and elastic bands to hold the wing on, a suggested way to find the location where the dowels should be placed is to place them temporarily under the fuselage and used an elastic band from one side of the dowel to the other over the top of the fuselage. With the two dowels strapped on in this fashion, the wing can then be attached in the manner that it will eventually be attached in flight. The dowels (and the wing) can then be slid forwards or backwards until a good balancing location is reached.