it's not necessary to tell until you're ready to be intimate. As far as them telling others, if you're worried that they may not be responsible about that sort of thing, then maybe they're not a good candidate for a relationship.
As to how to tell, that varies depending on your situation. In general, it's a good idea to have the STD talk about testing and so forth and that would be a natural entre for bringing up herpes. Be ready with some facts and maybe even a printed out copy of the Westover Heights free herpes handbook .
An approach relating to herpes that I've suggested before is to approach it like this...
You: "Say, have you ever had cold sores?"
Them: "Some answer. Blah, blah, blah."
You: "Well, I get them sometimes, but not in the usual place."
Then go ahead and explain that cold sores and genital herpes are both herpes simplex, etc.