Barometers are instruments that measure atmospheric pressure. The pressure is shown in the barometer's dial, and is usually represented in inches. While they are fairly accurate devices, you need to set one properly to get a good reading. The atmospheric pressure differs in locations based on elevation and weather conditions, so your initial setting will depend on where you live.
Find the pressure that your local weather station reports. Many stations will have the weather report on their website. You can also wait for the weather report to appear on TV.
Use a small screwdriver to turn the adjusting screw in the back of barometer. Adjust the screw so that your barometer matches the reading that your weather station is giving.
Turn the knob in front of the barometer to adjust the inside hand of the barometer. The outside set hand (the one you adjusted with the adjusting screw) will provide a point of reference. The inside hand will move up and down according to the current pressure; the two together will tell you whether or not atmospheric pressure is rising or falling.
When you take your first reading, check your local weather station to compare results. This will help you determine if you're getting accurate readings with your barometer.