Top the main cola when she hits 6-8 inches. The earlier you top the shorter the plant will be. Keep an eye on the secondary colas too. If they start passing the "top" cola go ahead and either FIM or top them too. If you are going with a bushy look you might want to get her in a tomato cage while you still can.
Also, you can help shape a plant by the size and shape of the pot. For short bushy gals transplant into a final pot that is wider than it is tall. Tall slender pots will grow a longer main stem.
Good luck, BigSteve.