I tell first time tokers EVERYTHING that might happen to them on a first time. They might feel nothing, they might pass out for a few minutes, they might start shivering/sweating, they might puke up. But, they probably wont feel anything at all.
If they smoke cigs, the best thing to do is take the baccy out of a ciggy and fill it with a WEAK herb/tobacco mix. They usually dont know the difference (I just say that its a ciggy from abroad) and then they will usually feel something for the first time, even if its a slight buzz. This is because if theyre holding a joint in their hand, they are expecting to get stoned, and for some reason when youre expecting to get stoned from a spliff you dont get much from it. When I get stoned, Ill just lie in the garden, eyes closed, and smoke the spliff without paying attention to anything. Before I know it Im fecked and theres still half a joint to go.

Oh, if you use the ciggy method, make sure they actually want to get stoned first, and take them someplace like a park where you can see all around you incase they get paranoid. Just chill