Login to your Authorize.Net account, go to Settings -> Security Settings -> API Credentials & Keys: here you can get your API login and Transaction Key details.
For Accept JS integration we also need Public Client Key, which we obtain automatically using API but you can always get it using Authorize.Net Merchant Interface: Settings -> Security Settings -> General Security Settings -> Manage Public Client Key.
Login to your AuthPro account manager, go to 'Integrations' -> 'Payments' page and click on Authorize.Net "Activate" or "Configure" button to open integration settings page. Put your Authorize.Net API Login ID and Transaction Key obtained in previous step, fill in additional information required and click "Update" button to save the changes.
You can create Authorize.Net payment buttons in your AuthPro account and generate HTML code to embed them in your website, Please note, button code is different for SIM and Accept JS integrations, so you'll need to get latest code and update it on your website if you change integration method or any other button parameters.
You can test Authorize.Net and AuthPro integration in sandbox mode, you can create Authorize.Net sandbox account here. In AuthPro integration settings enable sandbox mode and setup API Login ID and Transaction Key obtained in sandbox account settings. After complete testing, don't forget to change API credentials to ones provided in live account settings and generate new button codes to connect to real payment processing gateway.