Got a great premise for a movie, what now? Grab a napkin or paper and jot down your million dollars idea before it slips away (think J.K. Rowling). Let your idea simmer in your head for a few months and if it persisted, then you have something worth pursuing - you don't want to invest much time and effort upfront on an idea that may not take root.
Hire a screenwriter if you have the financial mean. Those on a budget or want full creative control, DIY is the best route.
The internet offers valuable resources to give you a head start. A favorite go to site comes from this Oscar-nominated screenwriter, Terry Rossio. With decades of experience in his respective field, included blockbusters like Shrek, Aladdin, Pirates of the Caribbean: Dead's Man Chest/At World's End and much more, he clearly knows what works and what not. His insight in the entertainment industry is unfiltered and honest and should help you with your concept development, story structure and the screenwriting business in general. You'll learn more by reading his blog than you would learn from a classroom. His columns are a must read.
Once you've mastered the basic in screenwriting techniques, download an app called Scripts Pro to your phone. Why this app? It's economical and ease of use and works on multiple devices. You'll be writing your masterpiece in no time. This is by no mean the best software but considering the low cost, it does its job well and the file can be exported to most professional software for further tweaking if needed.