If it is a 2 channel it will say something like 2x250w. You can tell by the number of screw terminals. One of the terminals will be power, one will be earth, one will be the remote power on from your headunit and if its two channel there will be four others ( positive and negative for the two speakers)
If you want to use a 2 channel just to drive a sub. You just use the positive from one set of terminals and the negative from the other set. Then you will have 1x500w to power a sub. It might also say this on the Amp.
eg. The picture above
As Dan said all you have to do is run an RCA cable from your headunit to the amp. This means(depending on the position of your amp) you will only need shorts runs of speaker cable. You need to run the rca lead on the opposite side that the power is running to prevent interference.
How this helps