You'll need a lot more than just a fan blade turning a motor.
As I understand it, you should check if you need a transmission system (one large pulley on the blade, one small pulley on the motor, a belt between)
From there, the motor acts as a generator, producing unstable voltage depending on the wind. The current is probably very low unless the turbine is large. (ever seen a wind farm and how massive they actually are?)
To be able to use this voltage, you need to regulate it. Connect the motor up to a DC regulator module that can take a wide input voltage range - you'll have to measure extremes before deciding.
That should output a stable voltage at x amps. X being unknown, dependant on the wind and your turbine.
Use that stable voltage to recharge or trickle charge a battery, which in turn powers another regulator that powers the Pi.
You also need to ensure the battery doesn't overcharge nor over discharge.
So you need:
DC motor, possibly geared or w/ pulley setup
Heavy gauge wiring
Solder (plus tools)
Wide input range DC voltage regulator
Battery (lead acid is probably your best choice)
Overcharge protection circuit
Over discharge protection circuit
Second regulator to convert the battery power, to the correct voltage and amperage for the Pi
A way to monitor battery state and cleanly shut down the Pi before over discharging the battery (it will not run 24/7)