When upgrading your Honda Civic, there are three basic things most people would like to do to gain performance, style, and looks. The first is a body kit. Next, most people want to add personalization to the interior. Last, we want to talk about engine modifications. These upgrades you may want to do in any particular order, depending on your personality. I started with my engine. I want my car to be a "sleeper". When I add a body kit, then I'll have the muscle to back it up. But it all depends on you. Let's start with the body kit for your Honda Civic.
If you have a pre-1988 Honda Civic, body kits were made, but they are hard to find. There really wasn't much of an internet, so the market was limited to larger cities or Japan.
Today, you could search for a used Mugen kit and probably find one on the internet. For later Civics, there are a couple of good choices. Stillen and Greddy offer quality components. When purchasing bargain components, remember you may have to modify the kit for proper alignment. For example, the exhaust may not be exactly in the right spot. Rework time to your Civic may end up costing you as much as a higher quality kit. I've looked at the kits you can buy a piece at a time due to my budget. Adding the front air dam to keep the car on the ground, side skirts and a rear valance all at once can be a little imposing on the pocketbook.
Interior modifications may not impress anyone as you drive down the street, but if the car looks good on the outside, you'll want that Civic sharp on the inside. A torn seat or other imperfections are a good place to start. Most people start with the radio. But if you have a newer Civic or Accord and the radio is already pretty good, what can you do to add your own style? Add some gauges. You can add a pillar mount gauges the A-frame that supports the roof. A company called Lotek makes nice gauge pods.
When you add the gauges, now you have interior style and you can keep better check on your motor instead of relying on a red warning light. The first thing to do is get a free flowing intake. Not only do these look good in the engine compartment, but also they help the car breath better. There are several to choose from, but I'm biased towards K & N Products. They fit perfectly and are completely washable. Be warned, you can here the engine breathing when you hit the gas! It's so cool!
I hope these gave you a good starting point on what you can do with your Honda Civic, Accord, or Prelude. Personalizing your car makes it your car, not just another Honda.