When your mare shows signs of foaling, bagging up, waxing, prepare her stall with whole straw. It is cleaner than sawdust or shavings which can migrate into the umbilical when it breaks. VSI Pet Care Products offers a foaling kit which I use and highly recommend. It contains everything you will need to midwife the foal. Don't be surprised if you spend hours watching and waiting only to have her drop the colt when you go to the house for coffee.
Mares can hold off labor for some time if they feel uncomfortable with the audience.
Assuming it is a normal delivery, your work begins after mother cleans her baby. If you are planning on imprinting your foal, do so in the first hour if possible, but not until mom has her time. After that, you will be busy with dipping iodine on the stump, enemas, and lots of loving. If the delivery is unremarkable, wait until a decent hour to call your vet. I like to have them come out and give an exam.