Harriet, since you are new to this try doing the shoot for free (no sitting fee charge) and just selling the prints that they choose. That way if you mess up you don't have to give anything back. They don't feel obligated and what they like they can purchase and that keeps you happy too. You get the experience, they get the photo session.

As for print pricing, best to ask around at your local photography studios to see what they would charge you to order prints if you were to use them to photograph you. Then price your accordingly. Are your prints the same caliber as they turn out? Then chagre their prices or just under, but don't undercharge for your services but also don't be offering any services if you can't produce quality on a par wiuth what established good photographers are producing.