Clouds are formed when air containing water vapor is cooled below a critical temperature called the dew point and the resulting moisture condenses into droplets on microscopic dust particles (condensation nuclei) in the atmosphere.
Dear Vignesh, if you understand terms like dew point etc. you can follow Aparna74004's answer but if you are in a junior class, this might be helpful.
Clouds are formed when water vapour from seas, rivers, lakes and other water bodies condenses to form tiny water droplets at a low temperature high up in the atmosphere.These tiny droplets then come together to form clouds. Hope this was helpful