To calibrate a thermometer, stick it in something where you know the temperature, and see if it's right. If it is not right, adjust it until it is.How do you do that exactly?
You can use ice and water: fill a glass with ice, add water almost to the top, stir constantly with the thermometer. You should end up with 32 degrees fahrenheit (0 celsius), because that's the point where water melts/freezes. With that much ice, but the water liquid, it should stay right at that temperature as long as you keep stirring (the stirring makes sure it doesn't get warmer spots or colder spots.)
Boiling water can work too, except that water boils at different temperatures depending on how high above sea level you are, so it will not always be 212 f (100 c)
And of course you need a temperature your thermometer can measure, not something too cold or too hot for it.