Day lays out some excellent advice here. I wish more people would listen to it.
Personally, I got started with casting. That's a great way to put your foot in the waters and see if you enjoy it. I found that I really liked it but after several months I felt that my popularity running the ScReddit KOTH's wasn't going to get me to a place that would pay for my bills. So we at the ScReddit team started Wellplayed.org. And while that was fun after several months of that I realized that the growth of that website wasn't sufficient enough to make it my full time living either... However their was this awesome site we used called Twitch. =) So the groundwork I laid out with my previous "failures" put me in a position to get hired at Twitch as a software engineer.
So my addition to Day's advice is to plan to fail, but plan to fail upwards. If your first endeavor fails to meet the initial goals, make sure you don't think of the entire decision as a failure. Don't put all of your eggs in one basket. Try your hardest to make everything succeed but put yourself in a position that if it doesn't succeed, you personally are in a better position to try again. And then keep trying again. Tenacity goes a long way in esports!