I don't even know where to start on this other than hiring a lawyer... How does one come up with Terms of Service for a site?
Goal of the TOS is to give a legal framework, so lawyers are indeed best to author/maintain it.
As a consequence, a TOS is required before you launch.
The TOS has a very high chance to be "hard to read" (with legal terms, etc.), and most people actually do not read it, and accept it anyway.
you probably also want to author (yourself) a Guidelines & Rules document, that will explain (in average people terms ;-) the DO and DON'T of your community. This content may point to the TOS.