In your nostr client, look for an option like Backup Keys, that's your private key.

It'll start with nsec. Your public key starts with npub.

Other people will tell you to use a signer app. You most likely will eventually. Don't worry about it today though. There's plenty of time to learn about the ton of other cool things you can do with nostr. Nostr can be quite overwhelming at first because it's way, way more than just a social media app.