The key is to develop passion, persistence, and patience. A good story takes time to develop. Characters need a writer to give them life and if you rush the process, you're inevitably going to be disappointed with the end result. One last piece of advice is to never write for money. That doesn't mean don't get paid for your work, it means if you're getting into screenwriting because you think it will make you rich, then look to other careers. The financial rewards are certainly there, but only for those who develop their craft first.