If you are setting a 2-step verification on Google, I believe the default option will be to send the code to your phone (SMS).
After you setup it, Google will give you another option to "Get codes via our mobile app instead".
On that screen, if you click on "Switch to app", you'll see a popup panel: "Set up Google Authenticator" with instructions to scan a barcode.
Before you scan the bar code, if you want to see your "secret", click on the link: "Can't scan the barcode?" and than the secret key will be displayed.
The google secret key will look like: "abcd efgh ijkl mnop qwer tyui uiop ab3c"