I'm 18. I want to join a startup and help it succeed just so that I can be a multi-millionaire by the age of 30. What should I do?

Before I give my actual advice .  Why do you want to be a millionaire at age 28?  What does that mean?  People will spend dozens of hours of research before choosing a bluetooth headset and hundreds of hours before buying a car.  But have you researched the likely results of becoming wealthy at a young age?  Is your life goal to find happiness, love, and useful work?  Or to be rich, dissatisfied, and miserable.  You CAN be rich AND find happiness, love, and useful work but the correlation between the two is almost nil.   What attitudes, habits, and skills will put YOU into that highly favored minority?

Now, some practical advice:  understand the concept of the "pit of success."  This is a key barrier to excellence and achievement in almost every field of endeavor.  Let's take piano lessons for example.  A child struggles to learn this one song and is very pleased with himself.  He moves on to something more difficult.  After struggling for a time, what does he do?  He stops trying the new song and goes back to playing the one he learned over and over.  The return to the comfort zone and what you can already do well might not stop your progress altogether but will slow it down.  You have to walk through that pit to climb the other side.