You need one nest box, about 12" on each side. If it opens in front, you will want a lip across the bottom to keep the litter or straw (or whatever you use) inside. You'll want to collect eggs daily if you can. You can put a golf ball or something similar --they are not fussy -- in there if you want, but they may use it anyway, at least after they have been laying a few weeks and adjusted to it. As a general rule they should start around 20 weeks, but that is very general, with a lot of variation. Usually the combs will redden and they will start squatting for you when they are about ready to lay.
If they should poop in it, I remove it in the morning; I keep a little hand garden trowel in the coop for this. You can close off the box at night til they start laying so they don't learn to sleep in them; I let a couple of them sleep there because I have a couple of crippled chickens. If they are already sleeping on the roosts, you may not have a problem with this.
There are quite a few articles in the learning center about the various aspects of chicken keeping.
