Chickens are often particular about where they lay their eggs. By this I mean that they usually prefer a private place away from other chickens when they want to lay. Chickens will often find a favorite spot to lay their eggs. This could be in a corner of your yard or in a corner of your coop. It is important that a nesting area be provided to avoid eggs being laid on the ground. No designated nesting area can lead to eggs getting dirtied, being stepped on by other birds, getting broken/crushed or even eaten. Nests to lay in will also minimize the daily egg hunt around the yard.
Nesting boxes are not something that have to be elaborate. There are some basics that have to be accounted for so that the chickens will find the nesting boxes more appealing than the dark corner of your coop.
First and foremost, they should be in a protected, dry place. Ideally, the nesting boxes should have a slanted top - the purpose of this is to keep other chickens from roosting over top of the nests and defecating on the top of the nests and the eggs.
Nesting boxes should also be filled with nesting material. Straw is a good choice and should be about 2-3 inches thick. This will allow the chicken to make their nest shape to their liking and prevent eggs from getting smashed and dirty on the bottom of the nesting boxes.
The boxes should be placed out of the main area of your coop to maximize privacy. Nesting boxes can be made to either sit inside the coop on the floor or attached to the wall ( 2' off the ground), or made to hang off the back or front of the coop, leaving the main floor space of the coop open for the birds to roam around. A good size for a nesting box is 1'x1'x1'.
If you are not the handyman type here are some good alternatives to wooden nesting boxes that work really well:
A 5 gallon pail turned on its side. Make sure you either attach it to your coop walls or brace it to keep it from rolling,
Milk crates can be an excellent resource for nesting boxes. Turn them on their sides and fill them with nesting material. These are easily cleaned and maintained.
Rubbermaid storage bins turned on their sides. Again - easily maintained and cleaned.
If you are really desperate and looking for a quick fix, cardboard boxes can be used. I would not recommend these as a permanent solution unless you are going to change them on a frequent basis.