So my buddies and I usually just have jam sessions with no vocals, well one of the guys wants to start singing and we got a mic.

whats the best way to use a mic w/o a pa system or whatever. Can we just run it through a guitar amp?
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Ermm.....You can but i wouldn't reccomend it. You can by like a Keyboard amp and put it though that. PA will sound best though.
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Not sure if there's any truth to this, but someone said running through a bass amp is better than a guitar amp, not sure if it's better than a keyboard amp or not though.
I would recommend a keyboard amp. I've done it and it sounds ok, might want a preamp of some kind though.
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PA is best, but not cheap and very awkward.

You can look into practice rooms with PA systems there, or using a guitar amp is a possibility, but it won't sound very nice.
PA best, Keyboard amp second best
If it has an XLR lead you'll need an adapter so it goes into the 1/4" stereo phone jack.

best way would be to run it with a small Beringer mixer though like the 502 which is about $30 new and useful for more inputs on a single amp. It has one preamp and XLR input.