Hi,
Your transformer sounds like it is center tapped (red=12v - black=0v - red12v) If you connect to a red and the black you will (from your description!) get 12vAC, If you use both reds and leave the black disconnected you'll get 24vAC.
If you have 2 1N4001 diodes handy then connect one to each red wire (Anode) then connect both diode's cathodes together and use this as your +ve then use the black center tap as your ground, This will give you a rough DC source but it's ample to run a fan on, This way you'll also get double the current from your transformer as you're using both 'windings of the transformer.
Paul.
When you used 1N4001 diodes and they didn't work then you must have connected something wrong, These are ideal diodes for what you need this time, I'm sure someone will correct me if I'm wrong but I would say these diodes are pretty much a standard when building a bridge rectifier with LESS than 1000mA load.