Honesty -- don't do this if you don't love it. There will be plenty of days when you'll wonder why you didn't just go into a nice, lucrative career as a ditch digger or something. On those days, the only thing that will keep you going is your passion for telling stories.
It's still hard, though. Maybe consider that ditch digging opportunity very carefully before you make the decision to become insane -- I mean, a writer.