If you are building your own plane or for some reason there is no indication where to set the center of gravity (CG) you should start with the plane a little nose heavy. Most wings that do not sweep a whole lot this method will set the CG pretty close. At least good enough so that it will fly without any problems. All you need to do is measure the width of the wing at the root (right by the fuselage) and set the CG 20% back from the front of the leading edge.
A common mistake is to assume that the CG should be set on the spar or the high point of the rib. This is not always the case, depending on the wing shape and such CG maybe optimal an inch or so in front or behind the spar. A plane that is too nose heavy is not a big deal to handle but one that is too tail heavy can become impossible to fly.
20% back from the LE is a good start but probably won't be the optimal location. You can get hints from your plane that indicate how far from the sweet spot you are. Get the plane trimmed out for level flight after the flight look at the elevator if it is siting slightly in the pull up (climb) position you may need to add a little weight to the tail. Don't move your CG more than ¼ inch before testing. Once the tail is sitting level you should be at optimal CG location. If the tail is in the push down (dive) position try moving the CG forwards. If the plane is hard to control in flight (doesn't track well) or porpoises this could be because its tail heavy.
Note: Tail positions either resting in the climb position or dive position could be caused by other factors like a tail or wing at the wrong incidence. So only adjust CG in ¼ inch increments. If the plane feels worse than before go back to the old setting.