Macleod410 wrote:I don't get why you'd put a mic on the 12th fret, sound comes out the body!
Condenser mic arms length pointing at sound hole is what I use.
err...sound is reflected off the top (and elsewhere). Aim a mic directly at the soundhole and you'll get a lot of bottom-end and not much else.
Mic placement is a huge subject and you could spend many, many hours researching this. If you have a single mic then, yes, pointing it at the 12th fret, about 12 inches away, is an excellent place to start.
If you have 2 mics (i.e stereo) than one at the 12th (small diaphragm) and the second (larger diaphragm) aimed at the lower bout below the bridge is a good place to start from. Also try a crossed-pair or X/Y, aimed at the 12th fret if you have two small condensers.
Bottom line is that you have to try a variety of positions and see what floats your boat; listen on decent monitors.
SM57s are great in front of a Marshall stack. There are probably better choices for acoustic guitar. Be careful, you can spend a huge amount on mics. No point in getting great mics without a great room...and a great mic preamp. I have been down that road (Neumann KM184s etc) and, after several years of frustration, ended up with an Apogee MiC and an iPad. It takes minutes to set up and tear down and - for a hobbyist like me - is perfectly fine and gives great results at a very reasonable price. I can then concentrate on playing rather than "the gear".
Not me but a cheap Yamaha recorded with an Apogee and iPad:
If you want to delve deeper into this stuff, get yourself over to Gearslutz, "low end theory" thread.