Learn about the kinds of fantasy stories or books you'd like to write. Read them. Read interviews with your favorite authors.
What parts of these stories excite you? What parts annoy you?
What (slightly unusual) words do you see in almost every book? Make a habit of using these words yourself.
Get inspired. Always keep paper and a writing utensil nearby. Write down every fragment of an idea. Find a spot that speaks to you and go there whenever possible, just scribbling down whatever comes to you.
Try to write every day. It doesn't have to be an organized journal. Just note whatever stands out to you.
For example, if you saw a guy with a strange deformity, what else did you notice about him? What was his body language and facial expression communicating?
Take bits of these things from your life and put them into your story. The fantasy genre requires much detail, and if it is something you've experienced, you can describe it in such a way that it rings true to the reader.
Who is your main character? Have a name for him/her. This character should be complex, with both strengths and weaknesses, and a history that makes sense. The reader should have a reason to like him/her.
Put all these ideas together. Put the ones that don't make sense in this context aside but DON'T destroy them! Do not be overly concerned with grammar, spelling, formatting, etc. Just write/type your story as it comes to you.