RE: How do I join wings to fuselages?
1st you need to have an alignment pin on each wing panel. This pin goes into the fuselage (or from the fuselage into the wing) to hold the wing at the correct incidence.
2nd fastening the wings. Depending on the size of the plane AND how you intend to fly it, anything from a couple of rubber bands stretched between the 2 wing panels, to bolts that go through the wing tubes to lock them in, to bolts that go through a hardened area in the fuselage into the wing panel will work.
Which one you use will depend on how the plane is going to be flown, the size of the plane and the amount of access you have for the method you chose.