Pylot,
First of all congrats for starting so young and having a strong internal compass. That will get you far, I promise!
The What right now is more important than the How. What = you realize you'll need a min amount of multi time to get hired on at a major. How = there are too many ways that you can reach that magic multi time.
Don't get wrapped up worrying about exactly what points a through z will be in your trek towards a major. What's important is that you stay focused on the very next step on your career ladder, and network network network.
Everyone in aviation knows and agrees with the saying that it's a small world here. It definitely is, and by staying dedicated to your career and making good contacts you'll get your multi time and desired dream job no problem..... by the time you're in your mid to late 20's, too!