there aren't exactly "stages" of mods, really. a good start would be maintenance. fluids, filters, spark plugs, distributor cap, rotor, check timing, clean the throttle body, etc...
after that, decide on what your goals are for the car. my suggestion would be a manual boost controller, boost gauge, brake pads, and maybe an exhaust. that's pretty general stuff that most people benefit from in almost any kind of racing or driving.
-Mike