one can never be too cautious.
Just becuase I know i'll feel paranoid in most cases, turn off your cellphone!! When I light up and the phone rings, either i'll forget i'm high and answer it, immediately regretting the action, or listen to the ringing and dreading what horrible things could be in the persons mind that's calling. Just turn it off and put it in your pocket.
Also: sometimes, if it's good weed and you're a noob, you can become very very very stoned. My first time with dank weed was like heaven. Almost dreamy. But it certainly didn't help to go out and rent a movie in this state. Realize that when you're high, you're not going to be overly conscious of all the consequences or difficulties doing certain things, so for your first time, just stay put. Don't wander off into public or ride a bike to a friends house.
Stay in one general area... and chill...