I want to build a quad copter. I want to know how do we calculate thrust or lift generated by using a motor, I am not aware about the capacities of motor. So can you explain how to calculate thrust or lift generated by assuming a motor. And what is the maximum payload that a quad copter can lift for a given thrust.
This is actually quite complex and depends on a fair number of factors. You will need to know the specs of the motor, especially kV, but also its expected current draw and the internal resistance. Obviously, your choice of propellers will also have an effect, and so will your choice of powerplant. It also depends on the ambient air temperature and pressure.
There are online calculators that can help you in that regard, this one lets you pick some common motor models and has a lot of other variables to consider. It offers feedback that includes lifting capabilities, engine efficiency and heat generated by the motors, among other things.
It should also be noted, that the theoretical values from the calculator can be quite different from the actual values you'll see in reality, but its a good starting point for helping you choose suitable equipment.