There's a lot of things to look at - the microphone's characteristics, and the guitar's characteristics, the playing style, the room you're recording in ...
For a start, try aiming the mic towards the neck end of the sound hole, about 12" back. Move stuff around until you hear what you like!
there's so many variables in recording an acoustic; the mic, the guitar, the player, the room, the set up, etc.... you're going to have to experiment to find the right position.
I'm no expert and don't do much micing on the acoustic, but the guitar and the room are probably the two most influential components for getting a good sound. i can tell you from what little experience I have that it ain't easy to get a good sound out of an acoustic.