Steve - there's a bit to learn and I suggest you practice with a light aeroplane in daylight!
Preparation - use the outside views to check your situation, what other aeroplanes are nearby etc.
Clearance - call up the ATC window and ask for taxi clearance. When you have acknowledged that, press the key for the visual path. If you can't see it from the cockpit use the outside keys, it'll be there - big pink blocks showing your path. Another way is to press 'Shift/]', which will call up a plan of the airport with your path marked on it (just press ']' to clear this).
Push Back (if needed) - press 'Shift/P' and the aircraft will slowly roll straight back (it's the equivalent of a tractor pushing you). When you're clear, press 'Shift-P' to cancel.
Taxi out - add power until the aircraft moves, then try to maintain a steady speed, not too fast, and just follow the path. Use the rudder to steer and the brakes to slow down or stop, of course. Obey any instructions from ground control, like stopping for other traffic.
Hold short - when you reach your assigned runway, stop at the double yellow line. In the ATC window, cancel the visual taxi path. The window should now offer you the option of contacting the tower. Do that, ask for takeoff clearance, then turn on to the runway and off you go.
I suggest you go to 'Flights' and try the first flight of 'Hawaian Vacation'. That starts with a very straightforward taxi out and takeoff in the Cessna, you should get the hang of it from there.
[Edited 2004-09-03 06:26:53]
"Once you have flown, you will walk the earth with your eyes turned skywards.." - Leonardo da Vinci