Replying to Amp/subs/car

well the other day while cleaning out my car from all the shit my friends leave in it i stumbled upon an amp under my back seat, i have no idea how many watts it is or anything and the dealer ship didnt say anything about it. As of right now i dont have any subs or anything in my car just stock shit but i was thinkin bout gettin 2 12's or maybe just 2 10's
how many watts would i need to acheive this and for how much?
help would be much appreciated