I am assuming that you have some characters in your head already.
OK. So here is the first step. Very simple:
Decide what you want to write about.
In one single sentence (about 15-20 words) explain what your story is about.
Example:
A young philosopher goes on a journey to find the meaning of life, only to discover that the answer is within.
Go ahead. Try it, it's not as easy as it seems.
Step two:
expand your sentence into one paragraph:
Write three sentences about the major disasters in your story and how they are resolved.
Another two about your climax and your ending.
Before we go onto the next step you must work on your characters.
You need to come up with a background for every major character in your story so they are believable... More info on creating believable characters is coming next! So start thinking and taking notes now...