Here are the steps to turn off test mode:
Log into your Merchant Interface at https://account.authorize.net.
Click Settings in the main left side menu.
Click Test Mode.
Click the Turn Test Off button. The interface will confirm that the Test Mode Settings have been Successfully Applied and you’re now in live mode.
Now you're ready to enter your Authorize.net API Credentials into LeagueApps. Follow Step 3 here to enter your credentials.
Here is a screenshot of the Authorize.net Test Mode page: