The first thing you need to do is to find the Mean Aerodynamic Chord (MAC)

To do this, follow the diagram below.

Once you have established the MAC, figure approximately 25 - 30% of the MAC for CG. This will give you a good starting place where a good pilot should be able to fly the plane.

Once flown, you can determine if you want to move the CG forward or back for your specific flying style.