If you're looking to get customers, don't underestimate the power of word-of-mouth. Perhaps give an introductory offer for the first lesson (who wouldn't pass up a discounted lesson?) to hook people in, or hold some sort of event that'll help make people aware your barn exists and is a possibility, e.g. If you've got a few horses and ponies available for lessons, hold a pony day where kids can come and have a little ride and learn something about the world of horses.
Look at your local advertising opportunities, and if you make a website, for goodness' sake make it a professional-looking one. As a consumer I am instantly turned off any product with a shoddily-made website.