Generally the interior and all pieces there should be painted firstly. It is because of the way the model is going to be assembled. Just if you put both halves of the fuselage together you can't do more right? The same may be for the wheel bays. Then the painting of the external surfaces. Also please note that the paint may work as the protection against the glue. As a result the correct sticking may be difficult or impossible. In the case the joint can be unstable leading to the getting apart of the model in time. Therefore you should pay your attention to the set pins and their socket also the attaching points etc ... A note to keep in a mind ... a glue for plastic melts not only the polystyrene but also the paint what doesn't look too good.
BTW .. if you want to get help please start your thread for the assembling in the Start to Finish section. I'm sure we can help you with resolving many dilemas and answer most of the questions. .. pictures are really welcom and helpful.