What I can use to put in the box

I assume you mean cockatiels? Here's a good site.

Breeding Cockatiels, how to breed cockatiels, cockatiel breeding, causes of cockatiel infertility and failure to breed, why cockatiels won't breed, how to candle eggs, nest box, nestbox, how to set up a cockatiel nestbox, what to use and put inside a (http://www.cockatielcottage.net/breeding.html)