After messing up box cutouts dozens of times I've found a rotozip tool
(or even a power drill with a rotozip bit) works well for cutting the
boxes right the first time. Just put the drywall up loose (about a
half inch off the studs is as far as you're gonna go for now) drill
right into the drywall somewhere inside the box (don't hit the box,
wires, children's toys or small tools like tape measures left in the
box for petes sake) then just run your bit around in the box quickly.
Then tighten the few loose screws, and finish the hanging.
Works on cieling too, but you've gotta put in a bunch of screws first
if you don't have one of those fancy lift things to hold the sheetrock.
If you've got a 1/4" straight cutting router bit, it might do the job
too...maybe... it might get gummed up a lot easier than a rotozip.
My last piece of amateur drywall advice: a case of beer and 2-3
friends are the best tools you'll ever have doing this job. Never
underestimate how fast a friend who has never sheetrocked before will
pick up the necessary skills to make it look decent, and it's a LOT
more fun this way.