1. Go to Google API Console and login with your Google account.
2. Now select project "Api Project" from drop down at top.
3. Navigate to "Apis & auth" and then click on "Credentials" you will be see a window now click on "Create new Client ID" button.
4. It will open a popup window, choose platform = web application and AUTHORIZED REDIRECT URI = http://example.com/(must be like(http://yoursite.com/)) then click on "Create Client ID".
5. It Will generate client id and secret. copy and paste api and secret in your admin settings option or use your api credentials