Approximately:

deltaV / Current

where deltaV = Supply voltage minus wanted voltage across fan

Let's say 13-8= 5V

For Current you'll either need to read the spec sheet, markings on the fan or measure it. Let's assume 200mA (0.2A)
This value of course will be at rated voltage, resulting current after applying the resistor will probably be somewhat lower - some experimentation might be required

R = U/I = 5/0.2 = 25 Ohm

Needed power rating: P = deltaV x Current = 5 x 0.2 = 1 Watt
I'd select the next higher value available, probably 2 Watt

/Michael