Basically you just have to find a yacht broker company and apply for a job there to learn the field. You don't have to get a license (unless you're from Florida or California, or not from the US). There are courses and certification programs that could be helpful, for example:
http://www.bcmta.com/bcyba/01yba/01certification.html
You can find more information here:
http://www.ybaa.com/
