You can read the car manufacturers recommendations for your particular vehicle in your car owners manual.
In the owner's manual you will also find the correct filter to buy to replace your old one. If you don't have your manual, don't worry. The store or site you shop at will be able to tell you what air filter size you need to replace your old one by the make and year of your car. It is important to purchase the correct size filter for your vehicle for optimum engine performance.
Open and secure the hood of your car, and find your air filter. It is usually found right on top of your car engine. Look at the new one, and then find that piece in your car. It could be in the middle of the engine, or a little off to the side.
The old filter is secured by a screw or a clamp, so remove this. Take out the old air filter. If you are new at this, compare what you have removed to the new filter to bought just to make sure. This may seem silly, but better to feel silly now than be sorry for using the wrong part later.
If the area is dirty you can wipe it off with a cloth, or a small car vacuum if you own one. You can also cover the top of the carburetor to make sure nothing gets inside.
Replace the old filter with the new one, put the cover back on, screw or clamp it into position and your done!