Well, I'm a singer in a band and need a PA of course but I can't afford too much right now. I got a mic, and a couple of good sized speakers apparently meant for old stereo systems. They have input jacks in the back, so I assume I could use them for my monitors somehow by running the mic into something and hooking up the speakers to it. Does anyone know how I can make this work?
wrong forum, but.. stereo speakers don't make the best monitors. if you've got an amp or PA of some kind, they'll work.. just maybe not as well as you're hoping.

but if theyre all you've got for now, go to a pawn shop or a similar place, and try to find a used PA/ powered mixer. that should give you enough volume to be heard at band practice, at least..
Umm, you'd need a mixer and amp... TBH home stereo speaker will be nowhere powerful enough, the impedences will be wrong as will the connectors etc. You cab do it on a budget but you can buy ready made PA systems starting at around £250 - £300...

Yeah, I don't think that the home stereo speakers will do much for you. I would say that your best bet is to look round for a pair of used, decent quality, active PA speakers (in other words, they have amp & speakers built in to a single unit). You can probably rig this up just with your mic for the timebeing and add a mixer and monitors later.