Whether you have the next "Citizen Kane" or not, your script could be thrown out if it is written in the wrong format. Screenplays can be broken down into three sections, a slug line, action, and dialogue. The slug line is the introduction to a scene and must be on every scene. It starts with either "INT." or "EXT." to indicate interior or exterior. The next line is the location and that is followed by either "Day" or "Night". Avoid sayings like "Dusk" or Dawn". An example would be "INT. LIVING ROOM - DAY". The action lines follow and describe everything that is happening. The dialogue is centered on the page and features every spoken word in the story.
One way to gain writing experience is by reading other scripts. A large collection of free scripts can be found online, but read them with caution because many of them are "shooting scripts," a very different format than the usual spec script. Use those scripts to learn about wording and dialogue, but do not copy things such as over-written action descriptions or camera angles.
The best way to write better is with experience. In almost all cases, your next script will be better than your last. You will grow and learn from the writing you have made and become a better writer. A hundred pages is a lot of content, and by doing this multiple time you will learn a flow and style that is unique and is your own voice.
One of the key screenwriting tips is to keep everything in present tense. A key part of this is avoiding words like "is" or words that end in "ing". For example, the section "John is running. He starts slowing down and is breathing heavily." Should be changed to "John runs, he slows down, and breathes heavily." This will make your script a lot easier to read and is the preferred way to write.
Concentrate on your story elements by using a screenwriting software program like Final Draft or Movie Magic Screenwriting Software. This software knows all of the screenwriting formats and can fill in the blanks for movie scripts, television scripts, and even outlines. Once you learn all of the shortcuts and tools, you will increase your screenplay production tremendously.