It's always a good idea to see a vet before you start agility, as well as find a trainer, as Flatcoatluver said. But after you get the "ok" from a vet, here's something you may want to try ^^

Lay the bar just flat on the ground. Get some realllllllly yummy treats (chopped up chicken, turkey, etc.) and put them right in front tof the dog's nose and slowly guide him over the bar. It may take a few times, but instead of being afraid of the jump, the dog will eventually learn that it is not so scary and that he gets lots of yummy treats afterward!