How do I add a contact form to my site?

You already have one. Every site is built with a Contact page carrying a ready-to-use form (Name, Email, Message), linked from the footer of every page. Messages go to Contact Forms, on the Messages tab. Open it in the page editor to change the wording or the fields.

To build another form: go to your site's Forms section (in the CMS sidebar), give it a name and add any fields in any order. Text, Message, Email, Phone, and Date (visitors pick from a calendar). Each field has a label, an optional placeholder, and a Required switch; drag rows to reorder and delete any field. You can also style the submit button (label, alignment, full width; colors follow your site's button style). Then place the form on any page with the Form block and pick it.

Messages land in Contact Forms on the Messages tab, and each form can also email you every submission, replying straight to the visitor when the form asks for an email. When Email is on for your account and the form has an Email field, visitors also get a join-the-mailing-list checkbox with the same confirmation step as your signup forms. Spam protection (an invisible captcha and a honeypot) is built into every form, and each form's messages download as a CSV with a column per field.