Intake or exhaust I would say. Some people say to do one before the other but I don't think it matters too much. I had a 2006 Civic (auto) as my first car and my friend gave me a Pilot muffler he was getting rid of; had it brazened on for a custom fit (again done by friends for nothing) and it sounded great. Not to mention gave me about a mile or two better gas mileage.
It's all up to you really but in my opinion I would go with a cold air intake (or short ram intake depending on your money situation and if the car is manual or auto as they don't make a cold air for auto) or some exhaust component (be it muffler or cat back) for a first mod.
First mods are pretty much playing around with your car and seeing where you want to go with it. There's a plethora of parts out there for bolt on performance, and it can be a little daunting the first time round. The pinned 'steps to upgrading' found at various points in these forums are good guides to start with. My advice? Work on your engine's breathing before you go anywhere else. Intake, throttle body, headers, exhaust, and a basic piggyback ecu is the route I went. I might also recommend the overlooked suspension/brakes area too for first mods. A sway bar (no more than $200) or two, and some rotors and pads are cheap mods that enhance your ride's capabilities rather surprisingly beyond stock levels as well. Don't forget, more power means harder stopping and wear on the rest of your ride.
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