The 'key' you used that starts with 'nsec' ...that's your private key. It can't ever be changed, so write it down somewhere & limit how much you enter it in apps. Damus & Primal have good reputations for keeping it safe, so you shouldn't have to worry there. Once you get situated, you can get a nostr signer app to keep it safe online & access all sorts of crazy stuff. Welcome to the amazing world of nostr 😊