Well if you want to hook up subs like the kind you put in a car then you need to get a power supply that can power them and an amp. With the power supply you have to make sure it has an on/off switch or else you wont be able to power on the subs.
If you are getting a speaker system for computers then you just need to have a sound card that is 5.1 or above. Basically it needs to have 3 or more ports to plug the speakers in.