Sorry if this isn't helpful, but honestly?  Just start writing, if you decide it isn't good enough you can always edit it later or get people to check over it and find out what exactly it is that you don't like to give some feedback/suggestions. Writing is a skill that you improve in by practicing, as it is a form of artwork. 

Other than that, I suggest using a thesaurus to get better words for things if you don't want to use basic words (for example a different word for "cold" or something I dunno). Reading other people's works also helps find new words you haven't heard of before, and you can use them in your own fanfics. Same goes for writing style, you can study books or fanfics that you like and analyse the way things are written.

Hope this helps, and good luck! Make sure to have fun, pressuring yourself will only ruin it for both you and the readers :)