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.
I know this is an old question and I hope you are well on your way in the auto sales industry, but for any new readers; It is possible to train someone to sell. Everyone, no matter how shy or whatever, can learn to sell. Most dealerships want you to have some sales experience before hiring you so I would suggest starting in another sales position first such as retail for a beginner. Yes, I know Auto Sales is a retail position, but it is a more advanced position than a lot of retail.
Once you have confidence in yourself and your ability to earn in a straight commission job, go to the dealership that you want to work for and ask for a position. Don't worry if they are hiring or not if you are right for them they will put you on. Remember, most auto dealers have you work on a straight commission basis. To get around minimum wage laws they may have you work on a draw where they advance you income that is paid back when you earn it.
Auto sales can be rewarding, but you need to be confident in you ability to live on a variable income and understand that the turnover of employees is very high. I worked on a 5 person sales floor that went through an average of 40 salespeople per year. If you make it in this kind of place you will be well compensated.
Good luck selling!