A Couple of Noob Questions

I have just germinated 3 Vanilla Kush seeds for my first grow and I have a couple of questions that I cant seem to find the answers to.

First off...I am wanting to use a PC cooling fan for circulation. I have read a lot about making sure that you box has no light leaks. How do you deal with the light coming through the hole where the fan is installed??

Also along the same lines. How do you determine how big the passive intake vent needs to be?

And finally at what point do you start the seedling on the 8/16 light schedule??

thanks in advance

RockLobster4040