Writer's Advice, given by Rick Riordan:

Most importantly: read a lot. You'll learn how to craft a good story by reading a wide variety of different styles and genres. The best writers are voracious readers.

Second, write a lot. Writing is like a sport. To get better, you must practice. Build your writing muscles by writing a little bit every day. Also, don't think what you write has to be perfect on the first draft. No writer ever born could write a perfect first draft. The main thing is to finish what you start. Then go back and revise, and revise, and revise. How do you tackle writer's block? Plan what you're going to write before you write it! Sketch out the main points. That way you won't get to the middle and lose steam (well, maybe not as much, anyway). Power through! It's always more fun and easier to start a story than to finish it, because finishing is hard work. I have two binders full of half-finished stories from when I was young. It was a long time before I ever forced myself to finish something.

Finally, Don't give up! I knew I wanted to be a writer when I was twelve. It took me seventeen years before I was published. My first book was rejected thirteen times before it was accepted by a publisher. Even after that, I had to be a professional writer for ten more years before I came up with the idea for Percy Jackson and was able to become a full-time writer! Very rarely, you'll hear about writers getting published when they are thirteen or sixteen or eighteen, but that is EXTREMELY uncommon. Most writers have to work much longer to perfect their craft. Don't get discouraged if you don't get published right away. Stick with it and don't give up.

You shouldn't even think about publishing or submitting a story until you have one finished. No one will want to talk to you until you have a finished product, because that's the first test you have to pass. Can you actually finish something? Anyone can have a good idea. Many people can write a few good pages. Very few can actually finish a manuscript. Once you have a completed manuscript, then and only then you should get a copy of the Writers' Market, available in many libraries and bookstores. This is an excellent how-to guide that will walk you through the process of getting published and provide a list of possible agents and publishers.