If the fan in your window air conditioning unit won't turn or if it's noisy, the problem is probably either dirty blades or a loose fan. Either way, fixing one or both of these problems is something you can probably take care of yourself.
First open the AC unit's cabinet and find where its fan is located. Clean away any debris with a soft cloth and and a hand-held vacuum cleaner. After the area is clean, you can now check if the fan blades are loose. If they are, you can tighten the blades at their connection to the shaft of the fan with your screwdriver or Allen wrench. If the air conditioner uses a round vent fan, you'll need a long-blade screwdriver to secure. Stick the long-blade screwdriver through the port in the fan and tighten the fan on the motor shaft.
If your fan has oil ports to keep it rotating smoothly, you can add a few drops of 20-weight nondetergent motor oil in each port. You should probably do this at the start of each cooling season, just to get your AC off to a healthy start. Do not use an all-purpose oil.
If you suspect that it's the fan motor that's the problem, and not simply loose or dirty blades, you'll need to test that with a volt-ohmeter (VOM). First, disconnect the terminal wires from their terminals. Then, clip the VOM probes to the wires. (Your VOM should be set to RX1 scale.) If the VOM reads anywhere between three and 30 ohms, your fan motor is good. If the reading is outside this scale, you need to replace the fan motor.
To switch your fan motor, first remove the fan, the power wires and the mounting bolts. You can then simply install the new fan motor by reversing this procedure. However, if your AC has the condenser coils blocking the fan motor, do not remove them yourself. In this case, you'll have to call a professional to do the job.