I'd recommend beginning at the beginning, so Volume 1. That way, you get the full story as the author intended without missing any crucial details or build-up.
I'd suggest beginning with the first chapter. It sets the foundation and introduces the main characters and the initial plot. From there, you'll be able to follow the story's progression smoothly. Also, check out some online manga platforms that have clear starting points and good reading interfaces.
Well, to read the JJK manga after the anime, first, you need to know the last episode or chapter covered by the anime. Then, find the next chapter and start from there. You can get it from official manga platforms or local comic stores.