If you just do an Internet search and you will see that there are more than enough references on how to write a script. Instead of going over the same information, it is more important to know how script writing will help your Brickfilm.
The best reason for writing a script is for practice. Most Brickfilm makers have high aspirations, and by taking the process seriously, it helps them keep improving on what is most important with anything you animate— telling a good story. Though animation is important, always remember that people are looking for a unique story.
Scriptwriting will help you in many ways. One is so you learn how to write dialogue. Dialogue should always move the story forward. This can be done by revealing plot, character, imagery, and many other things. If your dialogue is not doing this, get rid of it. The more you see the story happening in your mind’s eye, the more handle you will have when it comes to turning it into great animation. As you can see, many factors come together to making any story come to life.
There is a lot of software for script writing, both to buy and for free.http://en.wikipedia.org/wiki/Screenwriting_software