Don't waste your time. It's a Honda Civic. If you must do something to it (waste of money) then most people would say cold air intake ($150) and a new freeflowing exhaust system ($300). Seriously, though, despite what a lot of guys will tell you we are only talking about a few extra horsepower. You could also get an underdrive pulley ($200, sacrafices other things such as AC and stereo power, but not by anything noticeable). A new chip ($250) or new spark plugs and wires (cheap) would be good too. But seriously, a lot of these Honda hot rod guys will tell you a new muffler will get you 20 HP. They are full of crap. Unless you go all out and pay a lot of money for a whole list of upgrades, you won't notice a difference with just one or two upgrades. If you want to make your car more fun, invest in suspension upgrades. A nice tight suspension with minimal body roll and a stiff body will make a pretty crappy car feel a little more racey. Whatever you do, 5 years down the road you will be saying "why did I spend so much money on a civic?!"