You might first notice your hens looking a bit scruffy. And, possibly eating less. And then the egg laying will start to taper off. Don't panic! When your chicken starts to molt for the first time it can be a bit startling. But don't worry. It's normal for a chicken to shed their feathers about once a year in the fall (although they might skip a year the first year of their lives), to make way for new, shiny feathers.
The way it works depends on the chicken. Some might just look like they're having a bad hair day for awhile, barely losing any feathers at all, or losing them gradually. Others might lose most of their feathers seemingly overnight, leaving you with a rather naked looking chicken.
This process will generally last up to two months, but can go as long as four. Since it's just a part of a chicken's life, there's not really anything that can be done, or even should be done. Let nature take it's course. However, if you feed your chicken's little boosts of high protein foods (like shredded cheese) during this period, it will give them some nutritional assistance in growing back all those feathers. It's hard work.
You should also expect that egg production will stop until their feathers are re grown. The girls will be focusing their energy and efforts into replacing their winter insulation before winter hits. They won't have enough energy leftover for egg laying. This, too, is normal.
Eventually this whole process will be finished. Your chickens will look bright and shiny, like they've been visiting the day spa. Their old, broken feathers will be replaced with new feathers in good shape, helping them make it comfortably through another winter.
Here's more information on other chicken problems you might be interested in.