SM57s are a good first choice as they sound bloody great with guitar as well as working really well with some voices. My voice sounds better through an SM57 than through an SM58, the industry standard live vox mic.

Although SM57s don't require phantom power (+12-48 V DC) they don't have a very hot output which can lead to noise recordings. The solution to this is to use a preamp (even a Behringer will make a difference).

For recording classical, point the mic at the 12th fret about 12 in away.

If you are going to shoot striate into a computer it is worth remember Mic level signal is of a lower impedance than a guitar signal or line level signal and a DI box can make a huge improvement to the quality of your recording.

You can also get contact mics however when ever I have used them they have sounded terrible.