This seems to be a common question, so I'm going to attempt to give you guys somewhat of an idea of how to go about getting the right setup.
First off, before anything else, consider the type of music you listen to. Do you listen to mostly Rock? Rap? Country? How about Polka? All of these can determine the type and size of subwoofer you'd be best suited for. If you want punchier, quicker and clearer sounding bass, go with a smaller subwoofer, such as an 8" or a 10". For mid-type bass that isn't quite as boomy as a 15", but not as punchy as a 10". It is considered to be one of the most common types of subs used, for the fact that it can be well ranged if you pair it with the right box. Last, you have your big subs. These are the 15" and the 18". Bigger subs then this exist(21", 25", 30"), but for all practical purposes, 18" is the biggest you will EVER need. Kicker makes an 18" SoloX that's just ridiculously loud when used right.
So now that you've started to get an idea of what you'll be listening to, there's a few more variables to consider. Boxes. There are three main types of boxes you will use: Sealed, Ported, and Bandpass. A sealed box is generally a smaller box(uses less airspace in it's design, therefore smaller in size), and is generally used to deliver a punchier bass. Most rockers find that they are quite happy with the sound delivered by a 10" in a sealed box. A ported box has a port that allows air to flow in and out of the box. These boxes are generally louder then their sealed counterparts, but they can deliver a muddy sounding bass if not tuned right. The ports have to be tuned to a certain roll-off frequency(where the sound response level diminishes as the bass gets lower), so you want to make sure you don't shortcut yourself on that. The last and final type of box(for the purpose of this brief introduction), is known as the Bandpass. It's pretty popular with most dB draggers(it can usually squeeze a few extra dB's from a sub). It is a combinated of a ported and a sealed box. It has two chambers, and the subwoofer is mounted between the two. One of the chambers is ported, the other is sealed. I've never used one of these before, and I've been told that they will tear your sub up pretty quickly. They also run the risk of sounding like a one-note system, due to the narrow response band of frequencies that the sub will play. There are more types of boxes that are out there, as well as variations(called orders) of the same box. I could get into the physics of the soundwaves propagating through the box and the different designs you can use to maximize them, but that would more then likely bore the average reader. If you are interested, there are several excellent books out there that will help you out in that particular area.
There are two main types of qualities you look for in a subwoofer setup. They are commonly referred to as "SQ", or "SPL". Generally, SQ(Sound Quality), is sought after by the more discriminating listener. These subs have a quicker response, generally put out less bass, but they match the levels of the other components of your system(such as tweets and mids) and they won't usually tick off the people in the car next to you. SPL(Sound Pressure Level), as you may have guessed, is the opposite of what SQ is. It is generally referred to as "bone crushing, booming and frequently annoying bass levels." This is what dB drags are all about. Most people think they can just find the biggest amp, the biggest sub, and slap them to a bandpass box, and they've got it all. Those people are wrong. You need to carefully design out your box and figure out what frequencies you'll be listening to, the type of music you want, and the quality you're after.
Finally, you want to always consider your amp. This can easily be the most expensive part of your subwoofer setup. You want to make sure you buy an appropriate power of amp to match your subwoofers. You don't want to crank too much wattage to them, or they'll burn up. Generally speaking, a sealed box is more forgiving to subwoofers that are stressed by the amps, because of the fact that the air sealed in the box behind the subwoofer acts as a spring so that it is harder for the woofer to exert itself. Downside to that, is you will lose some dB's. There's so many things to consider in a setup if you want quality.
Sorry for making this so long, but there's just some important things that everyone should know about making a proper choice for subwoofers and the system that powers them. I realize how scary all these choices can be, so I've went ahead and made a little table for you to refer to as sort of a cheatsheet. =)
TYPE OF SUB BOX TYPE OF SOUND MUSIC
8" Sealed SQ Rock
10" Sealed SQ Rock
12" Sealed SQ Rock/Rap
15" Sealed SPL Rap
8" Ported SQ Rock
10" Ported SQ/SPL Rock/Rap
12" Ported SPL Rock/Rap
15" Ported SPL Rap
8" Bandpass SPL Rock/Rap
10" Bandpass SPL Rock/Rap
12" Bandpass SPL Rap
15" Bandpass SPL Rap
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To find out more about box design, refer to the page on box design on the12volt's diagrams page. Just look on your left. =) And as always, I'll be more then happy to assist you with any questions you might have as well. Just PM me, and I'll give you my email address so you can get ahold of me anytime. Sorry if any of this information seems to be incorrect, just tell me and I'll make the proper changes! You can feel free to make this topic sticky or whatever, I don't care. I just hope this helps out some of you newbies make the right choice when searching for the right subwoofer setup.
Hope this helps you out...
--mindctrl