Join an existing startup in the same field. Not only will you learn a lot - on someone else’s dime - but you will meet plenty of people hungry to do the same.
If you are an engineer, you'll just keep shipping and learn nothing and get paid peanuts while being told you're having the best time of your life working 16 hour days with no life. This is something I dont advise most people. If you live in a place like SF or NYC, unless you're a founder or in the first 5-10 employees, having significant equity, joining a well established company will make you a lot more money and you can still pursue your passions on the side.