If you haven't done it yet, unhook the battery terminals and clean the connections with a wire brush or sandpaper. Lights and stuff like that don't draw as much current as the starter. If the batt. connections are corroded a little they will still pass enough current for lights but not enough to turn over the starter, hence the starter will just click but not spin. When you unhook the battery, always unhook the ground side (negative) first and hook it up last. That way if your wrench hits ground while hooking the positive side you don't do any unnecessary arc welding. If the batt. connections are good its probably the batt.
