If your birds are at 12 weeks, you should still have them on a grower formula. They need the protein for growth. If you start layer rations too early, you can affect growth and cause other health problems, mostly from starting increased calcium at too young an age.
I use Purina Start and Grow, but there are plenty of other brands.
At somewhere from 16+ to 20 weeks depending on breed and maturation, switch over to layer feed. I start to phase in layer in small amounts at the start, and gradually use more of it and less grower, so I'm not really starting layer at 16 weeks so much as introducing it, and phasing it in ... also good for using up that last bag of grower. We're fully over to layer by about 18 weeks, though you might want to put this off a couple weeks if you have slower maturing breeds. The general idea is that their structure should be at the mature stage (though the bird won't be "filled out" before you start layer ration); ideally you want to get over to layer a bit before they come into lay so that they've already had the layer nutrition for a couple weeks when they start laying.
Here's the basic Purina chart:
http://www.poultrynutrition.com/OURPRODUCTS/FeedingChart/default.aspx
Start offering free choice oyster shell in a separate feeder at about 18 weeks as well, maybe a couple of weeks sooner if you have sex-links as they can come into lay earlier than other breeds. The oyster shell is for added calcium for egg shell formation. It's not really necessary if you are feeding confined flocks a layer formula, but if your birds are getting scratch, garden scraps, foraging outside the run, etc, then they are getting less than a full ration of layer, and may or may not be getting enough calcium from other sources.
I try to get mine to eat sufficient layer ration by feeding them only this in the morning, when they're hungry ... and not giving them scratch or free-range until after "breakfast."
Try to keep scratch grains down to 10%-15% of total, same thing with not letting them completely pig out on grass.
If you've been on crumble, and are planning to move to pellets, and also if you're allowing them to forage, make sure they have access to grit, either naturally from their environment, or offering free choice purchased grit. (Grit is small, rocky material larger than grains of sand, smaller than pebbles, used in the bird's crop to "chew" / grind up food).
I use pellets when I switch to layer because I think they waste less ... but it always seems to take them a couple weeks to get used to pellets instead of crumble, which is why I phase in pellets and increase them while decreasing crumb over time ... they always start out tossing the pellets on the ground, but eventually realize they are "food."
Sorry if some of this was stuff you already knew ... good luck with your flock.
