Getting a secured credit card is probably your best bet for actually building credit from scratch. You put down a cash deposit (usually $200 - $500) and that becomes your credit limit, so there's basically no risk for the card issuer. The key thing is to use it for small purchases you'd make anyway - groceries, gas, a coffee - then pay the full balance every month. After a year or so of on-time payments, most issuers will convert it to a regular card and return your deposit. That history of responsible use is what lenders look at later.
Being an authorized user *can* help, but it's less reliable. If someone adds you to their existing account with good payment history, yes, you might get a boost. But you don't actually control the account, so if they mess up payments or rack up debt, it tanks your score too. It's useful as a supplement - like if a parent adds you to their card - but I wouldn't count on it alone. The secured card gives you actual control and builds your own track record.
For a car loan in a couple years, lenders will care more about your payment history than a high score, so focus on being consistent. Set up autopay if you can so you never miss a due date. If you can, also try to keep your credit utilization low - use maybe 10 - 30% of your limit even though you're paying it off monthly. By the time you apply for a car loan, you'll have solid proof you handle money responsibly, which is what actually matters.