In the event of a fire, you should call 911 and evacuate the building! Your life is worth more than any property. However, if you are going to use a fire extinguisher during the incipient stage (small/slow growing/minimal smoke and heat) follow the PASS method:

Pull: The safety pin needs to be pulled out, don’t push down on the operating lever while pulling the pin, it won’t come out.

Aim: Aim at the base of the fire, the lowest flame closest to you.

Squeeze: The operating lever is above the carrying handle. This lever opens when you squeeze it down. When you let go, the valve closes and discharge stops.

Sweep: Sweep the nozzle back and forth to cover the entire base of the fire.

For more information refer to this link from the National Safety Council on Fire Safety