So, you have this great idea in your head? Where do you start?
First of all, if it's your first time, I would highly recommend picking up David Trottier's The Screenwriter's Bible. It's sold on Amazon. If you're low on funds check Ebay.
If you need software and you're on a budget, search on Ebay for screenwriting or screenplay software. ScriptPro is affordable (usually under $10.00) and works very well. It is what I used.
Some people will tell you to make an outline, use index cards, stand upside-down on your head. You have to find your groove. I sit and start writing. I edit later. If I have a dream that I can see on the big screen I take notes. Yes, I sit at my computer in the wee hours of the morning writing an outline for the movie in my head.
The one thing I would recommend doing before you start is writing your logline and synopsis. If you can't write those, you don't have a clear vision of where you are going. If you're not sure what those are ... Google them, now.
Here are a couple of blogs that I found very helpful:
Don't be surprised when people don't try to help you. I quickly realized that for the most part, this is a cutthroat business.
A hint ... even when you think you're done, you're probably not. Take a breather, read other peoples stuff, and pick yours back up later. It will give you a new perspective.
Another hint. Start every scene as late as possible while still getting your point across. End every scene as soon as possible while still getting your point across.