So I have my account set up, which should I do first?

I have my account created. I have a couple products I am looking at with manufacturers. One that I am ready to go with, but what’s next? Do I order the inventory and then set everything up on Amazon? Or do I set everything up on Amazon and then order the inventory? If so, what do I set up? Also, one product I want to move forward on is a children’s toy, do I need special permission to sell children products, or can I just go right ahead? Thank you