In the middle of the panel, nail about every twelve inches. Check local code for variance. It is advisable, if the studs are new wood, to double nail in the field. Hammer each nail until it is forced slightly below the surface of the panel - this is called dimpling. Also be careful not to ding the edge of the panel when nailing or handling.
Dings require extra mudding and finishing work.
If you are using drywall screws, be sure you screw them to just below the surface of the panel. But do not break the paper when you do this. If the paper is broken, drive another nail nearby to assure a good hold. It is advisable to have a special drywall hammer or a cordless electric drywall screw gun for speed and ease of handling. Place nails in adjoining sheets directly across from each other where they meet at a stud.