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.
I think there are two things I would advise for someone who wants to be a writer. The first is to keep reading—not just vampire books, but any and all books that even slightly catch their interest. Reading will open the world to you.
And, second, write a little something every day. It can be as simple as a long text conversation (but remember that when you’re sending in your first book, grammar counts!) or writing in a diary, or blog, or scribbling down an idea for a story. But the absolute best training is to try to write stories in a normal conversational style, to keep a blog that you update frequently, to write fanfic, or to write poetry (if you want to be a poet—or even if you don’t.)
I would say firstly and most importantly read as much as you possibly can, you don't have to read Harry Potter books, I'm not trying to flog it but only by reading will you get a really good idea of what, in your opinion, makes good writing.
You'll learn to recognise what doesn't work and you'll expand your vocabulary - always useful. After you've done that write about things you know - your own feelings and experiences - which is always a good starting point.
Resign yourself to the fact that you will not write something good first time, you're going to waste a lot of trees before you hit your stride and you will imitate people you admire first and that's fine - everyone has to start somewhere. And most importantly persevere - keep persevering.
My focus is the characters—that's the part of the story that is most important to me. I feel the best way to write believable characters is to really believe in them yourself. When you hear a song on the radio, you should know how your character feels about it—which songs your character would relate to, which songs she hates. Hear the conversations that your characters would have when they're not doing anything exciting; let them talk in your head, get to know them. Know their favorite colors and their opinions on current events, their birthdays and their flaws. None of this goes in the book, it's just to help you get a rounded feel to them.
I think outlining—in a very non-structured, free-flowing form—can really help. I didn't have to do that with Twilight, but it was very necessary for the other two books. I changed my outlines often as the writing led me in other directions—the outline is just a tool, not something mandatory that you have to follow.
Some of the best advice on writing I got from Janet Evanovich's website. She said if you want to be a writer as a profession, then treat it like a job. Put in the hours. Set aside time for writing, and then make yourself sit down and do it. Sometimes it's easy—the words flow and you can get a lot done. Other times it's hard, and you might only get one sentence done in an hour. But that's better than nothing.
Here's a tip that really helped me with book two and three: forget writing in order. With New Moon and Eclipse, I wrote out whichever scenes I was interested in, rather than starting at the beginning and working through to the end. I wrote most of the books in scenes, and then went back later and tied the scenes together. It cut out a lot of writer's block to write whatever part I was most interested in at the time. And it makes it easy to finish. By the time you get around to writing the less exciting transitions, expositions, and descriptions, you already have so much done! You can see a full novel coming together, and that's very motivating. (But you really need an outline to work that way--to keep from getting lost!)
A lot of people tell writers to write about what they know. And that's good advice, because it gives you a lot of things to draw on. But I always like to add that they should write about things that they love. And by that I mean things that fascinate or excite them personally.
The Hunger Games is full of things that intrigue me; you know, it's dystopia, it's got kids in it, it's gladiators, it's war, there are genetic mutations. The Underland Chronicles has fantasy, animals, sword fighting. And if you write about things that you feel passionately about, it is so much easier to write.