Qualifications
Clowns need a sense of humor (duh), good comic timing, and the ability to let loose and be silly. The last part is the most important one.
There are a couple of training programs to help clowns hone their skills. The most popular path to professional tomfoolery is Clown College (yes, that's a real thing). The Ringling Brothers and Barnum & Bailey Circus offers classes in several locations, and hold yearly auditions for admission. There's no cost to attend if you're selected, and most are offered a job with The Greatest Show on Earth upon graduation.
Clowns looking for prestige (we know—it sounds weird to us, too, but we've been assured it happens sometimes) should check out the National Circus School of Montreal, which offers an internationally-recognized circus arts program. While the school isn't free or even cheap, there are scholarships that could help cut the costs.