If we look at this diagram we can see the the area which a beam of light from the sun falls onto is much greater at the top of the earth than at the equator. This means that the the energy is less spreadout at the equator and so the sun heats the middle of the earth a lot more than the edges.

Heating the air at the equator makes it warm. The warm air rises. As it rises it cools and this means that the water vapour in the air condenses into water droplets, forming clouds. This air moves along to the poles where it descends. The cold air is then sucked back to the equator, pulled by the vacuum left by the rising hot air.