Hi Sherlock,
After setting up your form, you go to Module Config, Payment Gateway. Making sure you enable payment gateway to Auth.net, you can then go to the Auth.net payment gateway (check the test mode while still configuring and testing). You will need to have a merchant account with Auth.net and obtain your login and verification code.
Next, simply copy the example in the Additional Parameters box. Notice to the right of that are 2 columns. One is the Authorizenet Tokens and the other is the Field Tokens. These field tokens are the short field names you used on your form. Basically you're just telling auth.net that what they call &x_something is the same as your $(Shortname). When you get that all set, make sure to update settings.
To send emails, go to Form Completion Events and select New Completion Event. Name this event then select Dynamic Email. Select 'any field response' in the dynamic field.
We then go to the email itself, enter the from address. The 'Email to' should be the parameter with the short field name you set for the email field in the form. Check this against the column to the right and copy it exactly.
You can also use these parameters (tokens) in the body of the email to customize. For ex: Dear $(FirstN) $(LastN) will render as Dear John Doe in the email sent. Also expand the Advanced Email properties to see more options.
I hope these basics help you. Let us know if you have additional questions.
Candace