Hello and welcome! Creative prompts can be helpful if you're not sure where to start. You can look up something like the Ultimate Fantasy Novel Planner on Amazon if you want and it has some prompts that might get you started, or there are books completely filled with prompts. There's a book available for Kindle with 500 prompts for fantasy writers to get them started. There's also something called StoryForge cards that might be helpful for giving you the beginnings of a story idea. As far as any advice beyond that, it really depends on what style/subgenre you're interested in with regards to fantasy, as well as what kind of length you're thinking about. The process for a novella or a short story is very different than for a full-length novel.