I usually take the first top after the second set of real, 5-bladed leaves. I pinch them, but I'm sure others will tell ya to use a sharp instrument to cut them off. The object is to maximize the amount of canopy so you get lots of tops. When you pinch the top growth off, the secondary growth will grow out, and you'll get two tops where you pinch. Learning how to maximize this method takes a bit of time and experimentation. No one can actually tell you what to do and/or how to do it, though there's likely some graphic examples right here on this site somewhere?
This plant of mine, Fred, is only 11 inches tall, with a 24" circular canopy. Never had one so symmetrical, myself. Good luck. Don't be afraid to prune . . . it won't ruin your plant and you've gotta learn somehow!
Fred @21 days flowering today!