Each Nostr account is based on a private/public key pair. Your public key is your username and your private key is your password. You are responsible for keeping your private key safe, and it cannot be reset if lost.
While you can sign into a client by pasting in your private key, that method is not recommended - it's tedious and insecure. Instead, we will use a crypto wallet, which is a piece of software specifically designed to manage your private keys.
The Alby extension & lightning wallet has built in support for Nostr. It's a great option for new users, but you can also add your existing Nostr private key.
Visit getalby.com and click the "Install Alby" button. Install the extension to your browser.
During alby setup, you will be prompted to connect to a lightning wallet. If you do not have a wallet, continue with the alby wallet option and sign up for an alby account. If you already have a lightning wallet, you can connect to it.
Once alby is installed and connected to a lightning wallet, navigate to the alby settings page. Scroll to the Nostr section and click the "generate" button in the private key field. This will generate a private key and save it to your alby wallet. You can now use this private key to log into Nostr.
Now that you have a private key, you can log into Nostr. But remember, Nostr is only a protocol, not an application, so you will need to use a Nostr client to access the network. For this guide we'll use Astral, a simple twitter-like web client.
You are logged in! It may take a few moments for content to start loading. From year you can follow friends (by searching for their pubkey), view different feeds, and post content.