I want to write about fantasy novel, but my literary background is very low. What should I do?

Reading, reading all kinds of books, literary classics, magazines, excellent online texts (high-end texts, not white ones) will do. I can't stop you from watching online jokes, but it's useless.

Practice reading more, write a story you want to write, then revise it two or three times and refine the expression until you are satisfied. It's ok to write something else, anyway, it's just practice, so change it and practice tens of thousands of words repeatedly. It should be no problem, but if it doesn't work, it will be hundreds of thousands of words.

However, after tens of thousands of exercises, the literary level has basically reached the standard and the foundation is solid. Start writing dozens of chapters, and you will find that your literary foundation is getting better and better.