Well it sounds like it's too late for us to say don't use an agency, but the most important thing is to make sure you have somebody (whose primary interest in you isn't how much money you're going to make them) who knows where you are, whom you can call at the start of the booking and also at the end to confirm you've left and everything is OK. Don't give the agency pimps any of your personal details including your legal name or your address, and don't be pressured into showing face pictures if you're not sure you want to - there's plenty of time to decide.
As for starting the booking, just make sure you get your cash upfront and your security call out of the way and then the rest will come pretty naturally - you'll almost certainly be able to see whether the punter wants to chat a bit or just get on with it, and the key is really to go with the flow rather than try to work to a script although you obviously need to keep track of the time. Don't tell the punter you're new either - the vast majority are lovely, but that sort of news is like manna from heaven to the ones who aren't.
And do read the
main SAAFE site 
.