First of all learn the business. Motorsports is a business. If you are coming here and asking this question you don't have the knowledge or the personal relationships to make things progress. Driving skill is only half of the equation. You have to have the networkin and business skills to go out and find rides. With that said, it is as easy as the following.
Put together a car in a competitive class and win.
Win a lot.
Then put togethere a competitive car and team in a more difficult class and win, and win a lot.
Then, if you have won lots of competitive races and championships. Go out and find more money. Lots more. And put together a privateer race team in a meaningful series.
In the said meaningful series do well. Show you can win.
Now put together a program in a meaningful feeder series. After getting some experience. Win. Win a lot.
At somepoint if you really have the talent, and develop the networking and business sense you will find a ride. Talent won't get you very far in a career in motorsports. There as many true talents sitting at home or at desks doing day jobs as their are mediocore talents buying and financing their way into "careers" in racing.
Oh. If you haven't won anything by age 18. And really, won something competitive. You need to have huge sums of natural talent, ambition, and money.
Consider going to the UK and winning the Formula Ford championship. That would be a great start.