It all starts with water on the ground evaporating into the air by heat from the sun. then the warm air with water vapor rises up into the sky where its cooler until it starts into condensing back into small liquid water droplets, that are small enough to float in the air, which rises even higher condensing more and more into bigger droplets until it cant rise Anymore because of gravity causing the big droplets to fall. 

The reason why warmer air rises is that it is less dense than the cooler air above, so when it will rise until the temperature of the warm air evens out with the cooler air.

the beginning part where the tiny droplets first formed from the evaporated water, formed the base of the cloud, and the biggest size the cloud droplets can get before falling and being affected by gravity will be the top of the cloud.

The falling big cloud droplet will bump and coalesce with the smaller rising droplets becoming bigger and bigger faster and faster falling down until it leaves the cloud and falls as an individual droplet down to earth replenishing the water on the earth, which is called rain.

That’s the reason why the thicker the cloud is the bigger the water droplet size will be because the water droplet will have more droplets to coalesce when falling.

And the cycle goes around and around, evaporating from the ground, condensing into clouds in the sky, and eventually rain replenishing the ground with water.