Android app package name is a name of the package, you've specified when started a new project. You can find it in your Activity file.
SHA256 Fingerprint Certificate
There're 2 ways to find the SHA256 Fingerprint Certificate.
If you have already uploaded your app to Play Market.
Go to your Google Play Console > Release management > App signing.
2. If you haven't uploaded your app.
Run this command in the terminal:
keytool -list -v -keystore <keystore path> -alias <key alias> -storepass <store password> -keypass <key password>
You have to replace <keystore path>, <key alias>, <store password>, <key password> with your values.
3. If you don't remember your keystore values, you need to create new ones. Here's how: https://www.youtube.com/watch?v=HNbtFTtq-8o