Gone through Official Documentation: Authenticating Your Client
But still, either terminal was not able to find keytool or command doesn't work?
Following will give you SHA1, SHA256, MD5 for default debug key. And it can be used for developing and debugging with google play services.
Linux & Mac command
keytool -list -v -keystore ~/.android/debug.keystore -alias androiddebugkey -storepass android -keypass android
Windows
keytool -exportcert -keystore C:\Users\<USERNAME>\.android\debug.keystore -list -v
Replace with your windows account name example:
keytool -exportcert -keystore C:\Users\Aakash\.android\debug.keystore -list -v
If asked password, type: android