Proposal: History of Science And Mathematics
I would say the tags should/will mostly be about:
Specific time periods and locations: (ancient-greece), (prehistory), (renaissance-europe), (india), (nineteenth-century)
Specific subjects will likely be very important on such a multi-disciplinary site, to let people filter out topics they know nothing about: (abstract-algebra), (relativity), (crystallography), (genetics)
Topics: (first-proof), (biographies), (experiments), (scientific-data), (instrumentation)
Since the site does not yet exist there are no tags yet. Once the site exists every user with sufficiently many points can create tags, simply by tagging a question with it. Typically tags emerge as the questions emerge. Then discussions happen on meta to consolidate the tagging system.
The meaning of sufficiently many points is different for the different phases of the site. Typically it is, in private beta, 1 point suffices, so everybody can create tags, in public beta it is 150 points, and after that 300. See Reputation requirements compared
Editing tags, in the sense of changing the tags of a questions, is about the same except that the point limits are 1/200/500. Editing tags, in the sense of changing the name of an existing tag, can be done by moderators only.