First, navigate to setup->sponsors/advertising->advertisements, there you will have the option of adding a sponsor.

Click that to be taken to a form to fill out with your sponsor information.

After you have finished filling out the form, you can click add an advert/sponsor. On the next page you will see your advertisement listed, where you will have the option of editing or deleting it. To see the advertisements on your website, the sponsor must be a vertical banner and have an image/banner attached. Then add a sponsor module to the public site.