Need Help Buying Sub Woofers

i want to buy some subs but i know nothing about them. could someone answer me the following questions

is it ok to have 1 sub or do u need 2?

does size matter, if so what size could i get decent sound with, keep in my mind i just want nice sound while listening to rock music im not using it for rap.

what are the best brands.

can someone please help me

cmon u ass holes

maybe im just stupid...but what are subs???

ok thanks. i really want good quality bass but i dont want it to be so loud i cant breath like in my friends car

ok sounds llike you could do with a 600amp if you dont want it to be to loud i would get it in the 12inch
you will have to get an amp that suits the sub but make sure you get the right box to put it in this can make all the diffrence to the sound sony is an alright brand for the price.

an amp? my friend told me my 180 watt cd player would be fine. maybe im just not getting the term

lol well crap, maybe ill just get nice speakers this all sounds so expensive