From the Marketing Tab, Click on "Landing Pages" then select "Forms"

From here you'll want to click "Add New Form"

Once inside of the Form builder you can customize fields such as the Form name, description, what happens when a form is filled out and who gets notified, as well as any lists the contact can be added to once the Form is submitted.

Now once inside the form editor you can choose from a plethora of fields to customize your Form.

After the Form is completed you'll want to add it to your landing page now. To do that go to "Marketing" - "Landing Pages" - "Overview"

From here you can select a landing page to add the Form to.

Once inside of the Landing Page editor drag a form element onto the the builder.

From there you can select the form you just created

Now that the Form is on the page make sure to save your progress by clicking "Publish Now" or "Publish"