You go up the regular Mt. Whitney trail for about 20 minutes before you leave for the North Fork. The start of the North Fork trail is not that easy to see, even in the daylight, unless you are actually looking for it.
I would not recomend starting any sooner then 20 minutes before light, since the North Fork up to Lower Boy Scout Lake would not be easy for a first timer in the dark. My first time, it was not that easy to figure out....during the day.
If you do want to start sooner, ask around and tag along with someone who knows the route. Quite often we now start a few hours before light, hoping to get to Lower Boy Scout lake by first light.