If you don't get extra light and just put them in the window, they will start to flower. Then they have to go through this funky ugly revegetative growth cycle, which just wastes time. You are going to want to get them under light for 18-24 hours a day. I am partial to the T5 HO. Even though I have a couple of LEDs, I veg my smaller plants with the T5. I have a 2' 12 tube fixture that is my favorite.
I'm going to suggest that you do some reading so that you understand the life cycle of cannabis and its minimum needs. This will save you a lot of heartache and headache down the road. There really is a lot more to growing cannabis than say, tomatoes. Check out what medium you are going to want to use, what you are going to want to feed them, where they should be planted, where you're going to plant them etc, etc. Also, I don't think that you are going to want to try and grow 12 your first outing. Are you in a legal state? How many can you grow legally?