Such a simple seeming question, so many factors involved. Especially... what kind of mic is it (dynamic or condenser)?
Short suggestion: find a small room and line the walls with blankets or hang clothes all over it, etc. crank up the input trim level for your mic, and place it at a level with the soundhole of the guitar about 2 feet back and angled slightly up towards the mouth.
This however is very dependant on so many other things and may sound like crap. Sooo... a longer suggestion:
What kind of mic is it dynamic or condenser? Is your voice LOUD or do you mumble (or both)? Do you strum hard or noodle at it? What are you recording to? Do you sit down or stand up to play? How big is the room?
I'd suggest setting everything up ready to record and try a few different locations. Do a verse and a chorus (or whatever you call it, a softer part and a louder part) with the mic close up to the soundhole, closer to your mouth, way back in the room, a couple feet away, etc. And find how you like the balance best. Try to get the setting so it sounds decent during a quiet passage and a louder one. If you sing softly and mumble and whisper and it sounds great, then you hit the operatic Queen-esque chorus part it might kind of overpower everything. Moving your head around as you sing can help this... back away and turn to the side the louder you get, lean in and sing directly towards the mic for the quieter you get.
I did some like this with a nylon guitar and dynamic mic to a casette tape a couple years ago and the tape compressed it enough that the levels were fine. If you are recording to a computer you have to worry about headroom and clipping and maybe use a limiter to make sure that doesn't happen, or a compressor to balance it all out better.
For mine, I sat at my desk, placed the mic on the desk pointing at me, about a foot and a half away and at a height where the soundhole of my guitar was almost pointing at it, and the soundhole of my head (oh yeah, my mouth, sorry...) made a little equilateral triangle distance down at an angle into it.
A voice singing loudly is louder than a guitar strummed loud, and the tones it creates cut through a mix better too, so the mouth wouldn't need to be as close to the mic as the guitar for them to sound well blended 'mixed' by where you place your mic.
The problem is with a dynamic mic especially, that a voice sounds nicer and thicker the closer it is to the mic (louder too, but I'm talking about tone here, volume can be turned up or down later). A smaller room is better for this than a big one (I used a walk-in closet about 5x8 feet and with lots of clothes and stuff along the walls) because the microphone distance wont mean that the sound of the room is more apparent than the sound of the voice and guitar. Since you are recording at a distance from the source, your mic input trim level will be set higher, which means it will pick up more ambience from the background.
But anyways... babble babble babble... Just try it out at different spots first.
