when will my girls start laying?

It will happen -- be patient. Sometimes they will do a squat for you { if you had a roo he would be all aver them. } I'm not sure what the average age for PBR's to lay is - but they may also start hanging closer to the coop/ nesting boxes.

I had a feed store owner tell me that it was OK to give the pullets the layer food before they lay... but I have read differently on this site. { At this age I think it would be OK if they are near their average age of starting to lay.}

I started giving my pullets layer feed around the age of 17 - 18 weeks since some breeds start laying that early. 

Signs to look for:  The pullets will start investigating the nest boxes - may even sit in one for awhile.  When you reach out to pet it, she will "squat."  Squatting means she hunkers down with her wings out just a little.  It's what they do when a rooster approaches them - it's a sign of submission.  They may start doing what's called the "egg song" - which is just a particular way of squawking.  It's a "bawk-bawk-bawk-bawk- BAWK!" squawk.  Once you hear it, you'll know what it is.  Another sign is that their comb will get really red a few days to a few weeks before they start laying. 

ETA: Once they start laying you can provide oyster shell to make sure they're getting enough calcium - although layer feed includes calcium.  I still make the oyster shell available to them.

At around 18 weeks they will usually start to lay. The only thing you really need to do differently is to give them layer feed and oyster shells for calcium supplementation.