Write every day. Get your work critiqued by people who know how to write. Rewrite, rewrite, rewrite. Critique, critique, critique. Keep at it until you have the confidence to say to yourself, "Yeah, this is good. This is really good." If you're expecting to sell your work right off the bat, think again. Being able to write a marketable screenplay or a novel people will actually want to read will take time. Maybe a year, maybe two, maybe ten. Settle in for the long haul. But the great thing is...if you truly LOVE writing, the making money aspect shouldn't matter quite so much. Now go write!