Where to start?

Echoing what Maryn said, but I'd add a classic book on formatting and screenplay writing.

Many of the scripts you'll find online are shooting scripts, marked-up scripts, scripts by people who are a bit wacky but well-established, so they're free to break the rules they know well.

It will save you time and trouble to learn that a lot of the stuff you may see is actually not proper for what you'd be sending out and isn't what you want to send out to anyone. You want to be sure you've got correct length, arcs, action lines that don't go overboard, etc., etc. That sort of thing you can learn from a good class or from a classic book, of which there are many.

Reading scripts is an absolute must. But if you don't understand the basics of story, you may not pick up on precisely what the writers are doing and how they're doing it when it comes to developing plot, character, conflict and theme.

So I think some sort of primer, be it a class, seminar, reading books, etc. is also a must.

The real advantage to a class or workshop is that you can interact with the teacher and get questions answered and specific feedback on you're work while, or even before you write.

Take a class to learn the standards of format. Take a class to learn the basics of storytelling and drama. Take a class to learn the ideas of story structure. Read a few well selected books on writing screenplays and drama. Read The Poetics for the basic ideas of the structure of drama. Decide on what type of films you like. Pick you favorites. And then sit down and watch them again and again until you figure out why it works. Why you like them. And then look at the films you don't like. The films you don't think worked. And ask yourself why not. And what you would have done differently to pull them out of the fire. And then be thinking of your greatest idea all along. All of this will help you formulate your ideas and solidify what type of film YOU want to write and what type of film YOU believe should be made. And write it. Write what you are certain would be a film that you would be THRILLED to see if you had just laid down $12 or $13 at the box office. Write the film that would have you skipping out of the theatre thinking you just laid out $12.00 well spent.

And read other people's screenplays to find how they expressed themselves and how they dealt with certain situations in their writing.

Figure out story structure. Look at how they're all sectioned in kind of the same way: http://www.youtube.com/user/clickokDOTcoDOTuk/videos

Mike,
Here are some good places to start:

1. Visit the Writers Store website. Virtually anything and everything you could buy to help you is available there. Don't buy everything.

2. Get on the email list for SceenwritingU. Hal's free courses are likely to be too advanced for you at the start, but sitting in on a few of his free 90-minute teleconferences will help you see what you do not know.

3. Get screenwriting software and start the painful process of learning the software and what it does to help you with basic structure. If you really want to spend the money, get Final Draft or Movie Magic Screenwriter, but Celtx is free, and a few others are cheap.

4. Buy a screenwriting book and read. Save The Cat is very popular as a beginner book, but so are others: McKee's book, Syd Field's, and others.

5. Scour the web. But I suggest starting with these sites because they will give you an immersion into what real, working, professional screenwriters say about writing: scriptmag,com, creativescreenwriting.com, johnaugust.com (and there are others). Especially read what working screenwriters have to say about the craft. It's not that difficult to digest, and it will give you an idea of how far there is to go.

6. Take one of your stories and turn it into a short (12-to-20-minute, which is 12-to-20 pages) screenplay. The one about what dogs dream seems like a great story, but it has a core problem: rare is the good screenplay told by a narrator, and the visuals showing a dog thinking will wear on an audience by about page four.
7. At some point, attend a big screenwriting meeting. I used to hold the biggest one in the world, but it died. You might consider going to this year's London Screenwriters' Festival in late October. http://www.londonscreenwritersfestival.com/
8. If you go to the London festival, I can't advise you on which of the great Brits' sessions to attend, but do go to Pilar Alessandra's class or classes and tell her Bill Donovan sent you. She is a wonderfully supportive, very smart teacher.
9. TV, TV, TV,TV, TV, TV,TV. It is my firm opinion that far too many writers write feature scripts. The real money and the steadier income are in TV, on both sides of the ocean. And about one-twentieth of all the spec writers write for TV. I watch these wonderfully written British shows: Foyle's War and The It Crowd. I find Midsomer Murders a bit dragged out and flabby, but I like it anyway. I don't watch kids' shows, so I can't advise you there.
10. At some point, enter screenwriting contests and pitchfests. But wait 'til your work is good to excellent. Not the Malcolm Gladwell 10,000 hours in, because contests and pitchfests are both ways to sell and ways to learn, but at least a couple thousand hours in.

Of course, that's just my opinion.