Mapping software for defining areas

I need to produce a map with areas covered by local depots highlighted. At present, we have some ancient paper maps, so I'll need to do it from scratch. What I'd like is a zoomable map, complete with roads and all the usual stuff, with contiguous areas shaded differently. Of course I could just take a snapshot and overlay semi-transparencies, but I need to make it zoomable and portable, therefore file size is an issue, so it should be vector based. I have no idea how to even start such a project, or what software to use. I'd prefer Mac software, but Windows is OK too.