I would say just check around and look at stories by fanfic authors you like and you'll get it. The basic ones are first, giving a genre if you know it. It sounds like you're probably writing genfic (meaning it's not a romance with pairings) which can be helpful to put in the warning--not so much because it's a warning but because it will appeal to people looking for fic that isn't romance. Otherwise my guess is you're probably not writing something that needs a lot of warnings since there's nothing triggering--a lot of the things that get warned about imply veering away from the sort of subjects you'd see in canon. Like rape, cutting, anorexia--special subjects that people might want to avoid. And also a lot of warnings have to do with porn, and if you're not veering into that territory you can avoid a lot of those!

Some people warn for character death, but I've read plenty of stories that don't. It really depends on the story. If it's a story about a character death it's usually warned, but if someone's writing a long HP epic where there's a battle or lots of spying and killing going on I don't think they usually warn for it. That's very in keeping with the canon as well.