The client most used on iOS is Damus. Since Nostr is "bleeding edge", each client has its tradeoffs. They're all improving though! Sometimes I bounce between multiple clients (using the same npub, of course).

To tag a person, the best way is to use their npub in the note. Some clients will do that for you, after selecting the user from a list. In Amethyst (for Android), I type the @ symbol, then type the person's name, to display a list to choose from.

You can also make your own unique hashtag, like #katycatskittens, or something. Or use the existing "babechain" hashtag, that someone else mentioned here.