When to top depends on how many main colas you want mate. Top the first node and you get 2 mains. The second gives you 4, 3rd gives you 6 mains etc. I'm classing a node as a full set of leaves btw. We all seem to have different definitions as to what the first node is so just to clarify mine. I usually go for third to get 6 mains. The scrog net will take care of the rest.
Find yourself a decent scrogger to follow though buddy. Lots more people get it wrong than get it right but there's more than a few world class scroggers around these parts.