How do I learn about hosting my catalog within Coupa?
You need to talk to your customer about how they would like to access your catalog.
How do I implement a cXML/punchout on my website?
You'll need a dedicated cXML developer or a third-party consultant, for example, Punchout2Go, to make your website cXML compliant. Once your site is cXML ready, provide your credentials to your customer. They'll handle the rest. See http://cxml.org for more info.