Step 1- Determine the size that will fit in your car.
Subwoofers range in size from 8" to 18". As they increase in size, the box that it needs also increases in size. Certain subwoofers only work well in certain enclosures as well. Check the area you are putting your sub in for hinges, wheel wells, or other obstacles that may interfere with the install. Some trucks might not be able to fit deeper subs behind the seat.

Step 2- Choose an amplified or non-amplified sub.
If you already have an amp for your sub then you can skip to the next step. If you don't have one you'll need one. Bass notes need a lot more juice than the high frequency information in your music. To produce the low bass you can feel as well as hear, you need plenty of amplifier power driving your sub. You can get a sub in a box that has a built-in amp, so all you have to do is run power and signal cables to the unit.

Step 3- Choose the type of box for the sound you want.
Sealed Box- If you prefer a cleaner sound (more suitable to jazz, classical, pop, etc.), subwoofers in sealed boxes tend to produce tighter bass, allowing you to hear more of the sharp, punchy edges that define each note.

Ported or Bandpass Box- If you like rap, reggae, alternative or heavy metal, you'll probably want a boomier sound. You'll get this sound from a sub in a ported or bandpass box.