If you have no idea, it's a good rule of thumb to have the CG in front of the high point of the wing, depending on design. I had high aspect ratio planes that had the CG very, very far aft and flew pretty well, but that's an exception. If you put it in front of the high point to start, you will probably have something that won't be so unstable that it's unflyable. Having a CG too far aft can be, uh, thrilling on a first flight.