If you don't top you're plant will grow tall, and have one cola (that's the top), and that's fine if you have unlimited height.
FIMM? There's a good tutorial on FIMMing, by M Blaze. I'll try to find it for you. Topping gives you 2 tops, and lots of branching. FIMM gives you up to 4 tops, and better branching.
If you top of FIMM you're going to need lateral space for your pant to spread out. maybe 1 square foot or more per plant.
Node it where the leaves and branches grow from