The easiest things to do first are always intake, exhaust, and a chip.
A cold air intake, or even just a better flowing air filter, help your engine take in more, denser air, which makes it more efficient (more powerful).
Better flowing exhaust makes the engine more efficient too, as the engine exerts less force to rid itself of waste gases. Engines that have been modified in other ways already will benefit more from this, as they produce more exhaust. A stock engine will see less improvement, but still a few extra horse power.
Look for a performance chip for your car's model, specific to the engine you have. Depending on the chip, you can sometimes see big improvements in both horsepower, and fuel efficiency.
With your car moving faster off the line now, your gonna want to look in to better brakes soon too. What fun is all that speed, if you can't stop in time for that sharp turn coming up?
Larger rims will allow you room for larger brake discs, which have more stopping power. A set of performance shocks and sway bars will you keep you stable through winding turns. A sticky set of low-profile performance tires can do wonders for the car's handling, although you will feel more of the bumps in the road with them, same for the performance shocks.
