If you do a search there should be a lot of info on this.
Normal ideas are a GTI 8v Camshaft, maybe the GTI manifold, downpipe and exhaust. Setting up the pierberg carb so it runs nicely. Advancing the timing slightly to get more power.
A performance air filter may make it sound faster!
New plugs, dizzy cap and rotor arm, oil change and HT leads should make sure they are all up to spec as well.