It depends what You mean more specifically. You want to be writer to be the writer, or You really feel a need to create something You have in Your mind and make it more materialized? You are young, so just start writing. You will check your style and you will find out if it gives You any joy and satisfaction.
1. Write the thing You want to write, I assume You have some theme or idea in your head. So: take something as first scene, or start from dialog, or something exciting/interesting/controversial, something what would make the reader hungry for more. Give the "promise/glimpse" of what Your story will be about.
2. Reorganize it eventually, to make it more orderly. Often when You write, it will be chaotic at the beginning, because You want to develop a vision and test it first.
3. Give it to someone for opinion. But remember: don't ask anyone about technicall stuff or advices like: "Oh, here I would do this differently" except some literal mistakes, or the person that checks You is another writer. But I assume not, so You want feedback and what impressions this someone gets as reader. For example: maybe your tale is too vague and the reader forgets who is who or other details that might be important.
But at the beginning, just write and write. If it won't be giving You satisfaction in return, well... Then You will know.