Writing a script requires that you follow a very precise script format. The spacing of each line is important, but I am unable to give that formatting here. The font must be Courier 12pt.

It is often helpful to come up with a list of scenes that will make up the entirety of your film, and then put them on notecards. This will allow you to see the flow of your scenes and will make it easier to rearrange and cut them later.

Example:
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Int. Trees - Night Joe, Julie
He finally gets the girl.
"Why did it take so long?"
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Include whatever information you think will be helpful. The location, what the scene is all about, and maybe a key line that you simply must have. I have even color-coordinated the cards based on what characters are in each scene. That way it is easier to see what character has not been heard from in a while and the audience may soon start to forget.

Writing everything out in card/outline format will also insure that you have a complete story. Much like those hated outlines for papers, these outlines can save you a lot of trouble when it is time to rework your script. Plus, notecards are cool, so it won't be nearly as bad as a research paper outline.

I have found that writing out emotional or logical conclusions in a script is generally a bad idea.

Example: Mary looked at Roger, all the memories of their last 25 years rolling around in a cluttered way in the back of her mind.
Example: The dead bird exudes the thought of the shortness of life as it lies in the gutter, its neck broken.

If you want an image to mean something on screen, it had better be able to do so without the help of prose. So, a safer way to write would be to merely list the physical facts of the scene, and see if it says what you want.

Example: Mary looks at Roger uneasily.
Example: A bird lies dead in the gutter.

A note on dialog: Say it out loud.
When you are writing, it will help to say your dialog as your write it. This will help you rework awkward lines.

As a general rule of thumb, it is said that one page of a script will turn into one minute of film time. Thus, if you want a 90 minute film (a really short "full-length feature"), you will need to write about 90 pages. However, I would suggest starting small, and working your way up. One of my first scripts was 84 pages, and it was a pain to rework such a long story since I needed a lot of help writing. So, I encourage you to write a 5-10 page script, and see how that goes before committing yourself to an adaptation of "War and Peace" or "The Grapes of Wrath".