Hi, don't laugh, I'm female and don't know all the proper words, so here goes:
First ensure that all the 'wheels' next to the air vents are up as far as they will go. Then you have the three dials in front of you just to your left; (1) this is the one where you choose where you want the air to go - i.e. straight to the windscreen, to you and the windscreen, just to you, etc, etc. (2) the amount of heat dial, (3) the fan speed dial. Turn (1) all the way round to the last icon at the top of this dial. Turn (2) to its hottest setting (just to start off with and adjust when its warmed up), then make sure that the light lit up on this dial is the one to the right as you look at it from the driver's seat (not the other one, this should be lit up when you aren't using the heater at all). Turn (3) as far as it will go (again, just to start with) and you should visibly see the screen clearing as if by magic. It's normally very fast. If this doesn't happen then there may be something wrong with your system. Hope this helps a bit. :o)