If you're handling the format yourself then there's an introduction to the standard format here and a complete style guide in the tools section.

Screenwriting software

There's no doubt the easiest way to write screenplays is to use a dedicated screenwriting package. There are a considerable number available for all sorts of operating systems, but in the professional sphere the established leader is Final Draft.

The other big player in the industry is Movie Magic, which is becoming widely used. Either of them will easily cope with any screenwriting demand.

Using word-processors

Most people have a word-processor on their computer and are often comfortable with it. Most modern versions will allow the building of templates that achieve most, if not all, of the format requirements. Even more useful, the common types often have templates that other people have already built. In general my experience is that they work very well if you're writing on your own, but when you start to collaborate with others and share files the subtle differences between versions and tempates can be a bit problematic.

There are templates available for Microsoft Office, Open Office, Libre Office and others. Searching for them online isn't particularly difficult. If you find none of them meet your needs then adapt them yourself and share your version with the web.

Other software

There are plenty of other tools that might appeal, moving for the moment away from the screenplay format.

There are writing productivity tools that are primarily designed to help you develop story structure and plot development. I'm most familiar with Scrivener, which is inexpensive, supports Windows and Mac systems and makes the development of the story much more controllable.

There are also special software tools for story development, character development, plot structure and probably anything else you can imagine. Many of them encapsulate the lessons of traditional screenwriting teachers.