How do I calculate the temperature of hot air in kelvin?
You already know the temperature of the hot air in Celsius: 100 degrees

Using the equation Kelvin = Celsius + (273.15 degrees), you would get 100 degrees + 273.15 degrees, or 373.15 degrees.

I am assuming that this is a preliminary step that is required in order to find the temperature to plug into Charles' law (which must be in Kelvin), in which case you can use the formula I mentioned above for your other calculations as well.