To send someone an email after completing a form or landing page, you can set up a Sequence (which can contain one or more messages sent to subscribers) and connect it to the form.
Create a Sequence84 with your message(s) and ensure that you Publish it so that it's active.
Then you'll create an Automation Rule to connect the form to the sequence. Here's information on Rules81, and you'll want to set the Trigger to "Subscribes to a form" and the Action to "Add to Sequence" so that anytime someone subscribes via your form, they get added to the sequence to send them your message(s).