Fire extinguishers consist of a wide variety of ingredients for putting out fires. Some contain only water, while others contain foam. Still others contain dry chemicals. These leave a fine powder behind after use. This powder is not harmful to the environment, but depending on the type of dry chemical, it can be slightly corrosive.
Throw away any item covered with powder that is edible or cannot be easily cleaned. This might include plants or any type of food or liquid.
Brush the powder of off items that can't be washed off with water, such as paper items. Hold a garbage can under the item, and try to get the powder into the can, or simply let it fall on the floor.
Sweep up as much of the powder as you can, and throw it away. Alternately, you can rinse it down the drain or onto the ground. The powder is not harmful to the environment.
Use a vacuum to get the powder out of any cracks and crevices the broom could not reach. Check underneath and in-between items.
Wash any remaining traces of powder away with soap and water, or simply rinse with a hose. Check the ingredients of your fire extinguisher first, however. Some dry chemical fire extinguishers use mono ammonium phosphate, which can be mildly corrosive to metal. These are usually used in hospitals and labs. If this is the case, you will need to thoroughly scrub down any surfaces that had the powder on them in order to avoid corrosion.
Open windows and ventilate as much as possible before cleaning the area.