The other day I had a friend tell me she wants to write a book. She asked where she should go to learn how. I said just write it.
That might sound too simple, but it is the truth. If you have an idea for a story, then write the story. As it happens, my friend is good at spelling and grammar. Some people aren't. So then, turn on spell-check and have someone check your grammar... AFTER you write your story. Writing the story gets you going. Thinking about learning how to write that story puts it off indefinitely. So write, even if you don't do a good job with the spelling and grammar.
The thing is, there are two important parts to a good story - characters and how you use them. Characters need to evoke feelings in the reader. They need to either love them or dislike them (I don't like the term 'hate'). If you want to capture your reader, you have to bring them into the story with you. Using conversation helps with that. When you're describing something, use conversation to help you. And use action instead of passive acts. Yes, there are moments when you have to just out and out describe something, but for the most part you can use character action to show what you want your reader to see.
If you can write a story with believable and emotion-evoking characters who actually DO things and are not just described as doing them, then you will end up with a good story. So, turn on spell-check and get writing!
One other word to the wise... take constructive criticism. Don't be afraid of it. It is not about YOU, it is about your story. If you want to end up with the best story you can write, constructive criticism will let you get there. But ask someone who knows what that is. And don't take it personally. As I've said, it isn't about you. It's about you getting the best story possible out of your imagination. Sometimes that mans you need to let someone suggest ways to improve your work. We all need improvement, after all. Not one of us is perfect... whether we talk about ourselves or our work. If you can't take criticism, you don't belong in the writing field because you won't allow yourself to improve.
So, get writing, take criticism, and develop the best story you possibly can.