You cannot really be taught how to become a salesperson, which means there is no real course which can teach you how to sell; it is an art, an innate thing which can help you to be a successful car salesperson.
But the most important thing that can make you a salesman is your enthusiasm which no course can teach you. It should be developed by you, even if it is not intrinsic to you. To develop enthusiasm, you should take part in sporting activities, take part in the concerns of your area whether it is social or anything, develop a can do attitude in you and act according to that.
Since you are specifically interested in salesmanship of cars, you should have some technical know-how of cars.