Is she a mini or a full-size horse? Minis often have complications from being bred down in size, which is why I ask.
Both times my mini foaled, I happened to be there. Both times I was feeding at night and she started pawing, laying down, breathing heavy. I really believe she waited on purpose for me to be there both times.
If it's a large horse, it's best to just stand aside and watch, not get in her way unless she's having visible signs of distress. If you want, call a vet up and have them talk you through what to do on the phone. You have a very small window of when the mare foals, it usually takes between 10 and 30 minutes. Don't get in the way of bonding, this is very important. Let the mother lick the sack off and the foal, this is imprinting and is how they bond. Make sure the foal eats within the first 12 hours, the colostrum is needed so they get the necessary nutrients. SAVE THE PLACENTA and take it to a vet within a day after foaling, so they can make sure it's all there and nothing is still in the horse that could kill her.
Find some books and internet sites, watch some foalings on YouTube, become accustomed to how everything goes. Stay calm, and remember to call a vet for support if you need :)