Ask your internal supervisor what they use for this kind of diagrams. No need to do anything other than the company standard, I'd say.
https://www.google.com/search?q=management+information+system+architecture+diagram starts with a nice picture showing 12 different diagram types. And surely, if you go to the library of your local university, you'll find many more in the books.
I am working as a first time Information Systems Consultant. As part of a report that I am going to send to client, I would like to include a diagrammatic representation of a proposed management information system, a site map of what the system will look like an contain. I am looking for advise on what sort of diagrams are best to do so. Please help..