We do not use the concept of "AN" in ASL. Suppose I wanted to express the concept of "I ate an apple yesterday," I'd sign "YESERDAY ME EAT APPLE."
If for some reason you need to indicate "an" you could spell it. For example, suppose you were helping a friend fill out a grant application and she had put the word "and" where proper English calls for "an" you would simply spell the word "A-N."
American Sign Language University ™ ASL resources by Lifeprint.com © Dr. William Vicars