The goats should be at least 6 months old. You can test for anything you want annually. Just depends on how much money you want to spend. Most people test for CAE and CL. But plenty of people also test for Johnes and Brucellosis.
You can either learn how to draw blood yourself or have the vet do it. You could even have the vet out the first time and have him show you how to draw blood to do it yourself later.