The best feed for chickens is Layers Pellets.
This will ensure that your hens lay a lot of eggs, while also providing them with a complete diet.
You can also give them some mixed corn, but only on occasion as a treat otherwise they will lay much less.
Chickens tend to eat about 150g of layers pellets each day, but this may vary.
You can leave the feed available to them all the time, they generally will not overeat.
It is important to make sure that your chickens have plenty of access to fresh water as well.