By default, Pardot forms display the specified Thank You content immediately upon submission, without sending the visitor to another page. If you'd like to redirect prospects to a particular page, you have two options: redirecting at the form level, or redirecting at the landing page level.
Redirecting at the Form Level
There is a option on the Form Wizard under Completion Actions that says "<label for="is_use_redirect_location">Redirect the prospect instead of showing the form's Thank You Content</label>". When this box is marked, a redirect URL text box will appear for you to enter the page you want the visitor redirected.
You could also redirect into a new browser window and control the size of the pop-up with something like the following:
Redirecting at the Landing Page Level
If your form resides on a Pardot landing page, you can also redirect prospects to another page at the landing page level. This option is available at the bottom of the Content step of the Landing Page Wizard. See Landing Page Scenarios and FAQ for more information about redirect scenarios.