First Microphone set up (help)

So my plans are to buy a microphone and start practicing with it. I talked to some guys at guitar center and they suggested I buy a PA and If I didn't want to do that I could put the mic through a keyboard amplifier.

My problem is, I'd rather spend cash on guitar equipment than a microphone set up. I already own a Line 6 Spider III 30 watt practice amp. I have heard that some people have gotten decent sounding voice out of it with a mic plugged into it. Can I do this? Is there a certain kind of mic I should get for this application? Will I break a mic if I put it through a guitar amp and sing through it?

You should be ok if you keep the volume relatively low although, you won't get the greatest sound if you're going through the same amp you play through because your vocals will be competing with your guitar all the time.

My advice would be slightly different. Rather than get a P.A. you might be better off getting a decent mic and some basic recording software.
That way you can actually hear how you sound and work on your vocals and other things like harmonies.