Start with small things and work up. If your not sure when your spark plugs last got changed swap them. Your easiest and most cost effective stuff would probably be your air flow system. Intake, header, exhaust. The whole thing not just the muffler.
That should give it a decent kick. You could upgrade the brakes too, if you love your car and plan on driving it in a "spirited" manner they will be helpful. You don't necessarily have to swap out the entire assembly or get drilled rotors, just get good pads to start.
Once you get past the small stuff you can always upgrade your internals, fuel injectors, pistons, rings, gaskets. If your in a manual you can upgrade your transmission with new parts as well. Doing that is sort of like upgrading brakes though, it won't make you any more powerful but you'll be able to control it better.
If you just have too you could boost it or get a supercharger. Both have positives and negatives and those are for you to decide. To do this properly though you will need to hit the first steps, otherwise you'll get just a small amount of boost and it wont be a cost effective mod.
Just please don't strip out your interior. Please. You will gain literally nothing from doing so and just drop your cars value and comfort lol.
