Cutting the end off the charger where it goes into the phone, should have either 2 or 4 wires. Assuming they are colored correctly it would be red, black, green and white. Ignore the green and white as they are for data.
I'm guessing your computer fan has the 4 pin setup. Red, 2 blacks, and yellow. The yellow is +12volt, red is 5 volt. Your cell charger is likely 12 volt but that should be on the plug.
Please be careful, and check the transformer's output. I never did this myself but I believe it is correct.