What subs are decent and cheap?

I need to know how subs work. like do you need an amp. what kind are the best. where can you get cheap ones?

• my friend has some subs in his car and they sound amazing. my sound system sucks and i need to know what to get. do you need speakers and subs. how does all this work out. someone please explain to me

• Answer:

  if your speakers are fine but just dont have bass, then buy a sub woofer, a sub enclosure (box) and you will definately need an amp strong enough to run the sub. If you want improved sound quality, you need new speakers