What kind of planning should I do before I start to write a novel?

Beginning Writer's Answer Bookedited by Jane Friedman

Planning methods vary greatly among writers, who often modify their systems
as they gain experience and maturity. Some are satisfied with a brief summary of the plot, the character’s conflict, and the resolution. Others may put together as many as fifty or a hundred pages of detailed charts of action, character, and environment sketches. (This is usually referred to as outlining.)

Writers who advocate outlining say that it cuts down on wasted time and endless revision because you understand your characters at the outset and can produce a logical plot. But writers against outlining say it inhibits creativity and inspiration, and even prevents them from finding the true line through the story. You’ll have to decide for yourself what makes sense for you. Perhaps it would be useful to begin with an outline but give yourself permission to go off script and revise your original outline if you get new and better ideas once you start writing.