i want to do something to my 2001 celica gt

Well the two things that pretty well any car should have done is intake and exhaust. I would start off my putting a cold air intake, then a header and high flow cat. If the header uses bigger pipe then your exhaust i would change it to match the header pipe size all the way back. This way you are getting a good flow in and out of your engine and give you a good base for future mods. Next I would upgrade your ignition system. Now I do have a lot of automotive experience but I am not familiar as to if your car has individual coils, a coil pack, or a distributor. I would upgrade the individual coils/coil pack. If you have a distributor and coil then i would change to 8mm(or thicker) spark plug wires and upgrade the coil as well. This will give you a better spark and a better burn you might not gain much if any ponies but again this is setting you up for future mods. Personally I have a distributor that I have upgraded with a bigger coil (15,000V -> 45,000V), 8.5mm spark plug wires, and changed the from ignition points to electronic ignition(they stopped using this in the 70's). These are all pretty basic mods that you can probably do at home with the right tools... maybe not the exhaust but the rest you can. The last suggestions is pick up an repair manual and check out some online forums. there are lots of knowledgeable people on them that are usually willing to help and sometimes you can find some good deals on parts.