One of the most important things when writing a story, in my opinion, is having clear what is the story about, why happens,when happens, who makes it happens, etc.
Then, work on characters, a lot. As a future writer I have found myself having troubles for making good characters. They must not be similar to each others, they must have quirks and must hve great things. You must know how to make people hate them and love them at the same time. They must be tridimensional, round and have a growth. Of course there's famous stories (and quite good) with flat or schematic characters, but I really think the better ones have this type of charcters.
Also, don't start using your story, wait, practice, make plot lines and when plot lines are made, search the holes. There is any hole. OK, don't worry, just think about how can you make it better. Now, there is any hole? Great. Then you maybe ready!!
Edit: Almost forgot. Also, do reasearch!!! You must know if you're going to write about something you didn't experienced