Finding the center of gravity/centroid of an airfoil?
In addition to checking out wing frame structures on the internet, why not go down to the hobby shop and get a balsa plane kit?  Airplane building is a hobby you will have all your life.

Generally, in balsa, you have a top and bottom spar, a leading edge, and a trailing edge to attach the aileron (get lots of sand paper and CA glue).  As mentioned, put you spars at around 25 - 33 % from leading edge, around the highest part of the rib.  You may also have good luck strengthening your wing by covering it with 1/16 sheet balsa, others use heat shrink monocoat.  But you must also stiffen the wing so it can not twist when you use the ailerons.  So you may wish to add balsa between the ribs connecting the top and bottom spars, creating a box structure.

A lot of ways here, and you can't read too much about it.  Please let us know how you are doing with it.  Check out RC Groups website as well.