here is a good list of starter mods, fisrt off you need to get familiar with how a machine like an engine works, the 99 camry's two engines that are available (I4 cylinder 5S-FE or the V6 1MZ-FE) are not simple machines and you will run into problems if you dont know what you are doing, so i recommend buyng a haynes manual and reading through that a bit, then get under your hood and get familiar with the engine
Fortunatly for you there are some parts available for a 99 camry, if you have a v6 engine you can get a supercharger but uhh, i dunno how deep your wallet goes they are pricey
here is a list of mods you can do when you know more about how the engine works
-get a new better flowing y-pipe installed
-get rid of your cat, or buy a high flow
-clean your intake, this involves removing most of what you see on top of the engine and then cleaning it, this will help a decent amount cause let me tell you the exhaust recirculation system dirties the intake a huge amount!
-make your own cai, this wont add too much power but the sound is amazing! you can use a variety of thing to do this, i used some 3'' aluminum electrical pipe i found at lowes and completly removed the intake from the throttle body all the way into the fender, there are a lot of resonators and junk that can come out and just run the intake into your fender, not that hard
-kill your egr system, you will need to block this off at one or two points along the system, the egr (exhaust gas recirculation) can be easily disabled if you end up cleaning your intake cause you have to remove the plenum, there is a pipe on the back that should be copper or brass colored, all you do is detach the plenum and before you put it back on fabricate a piece of metal to go in between the pipe and the plenum that will completly block the flow of any gas, this WILL trigger a CEL (check engine light) on your engine so i dont recommend doing this
-port your acis (v6 only), this will help some and its easy to do! do a search in these forums for FREE V6 MOD and you will find detailed instructions on cleaning your intake and porting your acis, though if you have a 4 cyl camry the procedure will be different because those instructions are for a v6
good luck