Easy way to grow hair out is to start with a regular short on the sides longer on top cut, basically ask for a "classic taper." If your hair is pretty short to begin with then you'll have to let it get to a couple of inches on the top.
The taper will be a good base for growing it out, but make sure you don't get straight lines on the back of your hairline. You'll want that blended/ tapered as well as it helps keep the neckline looking clean longer. Also, make sure that your barber/ stylist is able to get a good blend as the long hair on top transitions to the shorter hair on the side.
After that just let it grow and shave the back of your neck every once in awhile. You can go in for a trim from time to time as well, just get the same cut, but don't let them take much, if any off the top. This will help keep the length in proportion.
I know this because I'm pretty lazy about getting haircuts and have found this to look decent as it grows from short to long.
Of course if you want to be long all over from the beginning then disregard what I've said but you run the risk of unwittingly rocking a mulletish head.
