Pick one of your ideas, Imagine it's already a movie. Are there people standing in line ready to pay twenty bucks to see it? Or is it just kind of an OK idea? Pitch the idea to friends and family, ask if they would pay to see it. They'll probably be nice and lie, but you'll know by their reaction if you have a good idea or not. Don't start until you have a big, good idea. And make sure it has a beginning, middle and end. Pick one of your favorite movies, break it down into beginning, middle and end. Then make your idea fit that pattern.
Next, write the logline before you EVER start writing your script. If you can't say your movie in a line or two, you don't know what it's about. Go find the logline for "The Gladiator," "The Sixth Sense," etc .... Make your logline only hint at your story and peak interest like those loglines do. Then use your logline as a compass to make sure your script stays on track while you write it.
Now the fun part ....
Write out ALL the great scenes that you imagine your movie having. Ya know, the reason you love your idea in the first place. Hopefully you've got at least ten.
-Luke Skywalker meets R2D2 and C3PO
-Obi Won gives Luke his father's light saber.
-Luke proves he's more than a farm boy in his first aerial battle
-etc ....
Once you have these, start thinking about what order they should come in. Arrange them then start filling in the blanks, tying those main ideas together. And don't forget to make sure it still matches your logline. If it's better, re-write your logline. I say this because your logline might sound like a comedy but your actual story comes off like a bloody Tarantino movie. Not that that's bad, just make 'em fit.
Once you have that, you more than half way done. Let's say you do have ten great ideas, all arranged in the most satisfying order. All you would need at that point is nine pages for each idea - that's your script, 90 pages.
Then, once you've written it (hold onto your butt) re-write, from scratch. Make it better, make the dialogue better, more realistic, then write it again. After doing that about five to ten times, you might just be a screenwriter.