How do I start a career in racing?

No money = no motorsport. It's really quite simple. You can only start in motorsport if you have a decent bit of money behind you. And at 18, you're far too late for F1. Did you know Danill Kvyat, who drives for Toro Rosso, is only a year or two older than you? And he certainly didn't go from beginner to F1 in those two years. Rallying is less age-oriented, but again, no money, no go. A rally car is a highly specialised piece of equipment and then there's licences, entry fees, other equipment...not cheap.

F1 is essentially impossible for you i'm afraid. If you had started between 10-13 years ago, then yeah you might have been able to get into F1. but 18 is way too late, unless you are a billionaire, or unless you are some driving god that's better than Senna, Alonso and Schumi combined. But you can always get into F-1 as an engineer or mechanic? Like Adrian Newey, you can dsign formula one cars if you want

You really should have started 10 years ago. Without financial backing I would look for a new career.

you cant just go straight into F1 you have to do like everyone else start at the bottom and work your way up most if not all F1 drivers started in go karts