Where does lightning come from? How does it happen?

Lots of information about this is covered on the following site. Lightning - Wikipedia, the free encyclopedia

It's useful to start thinking about lightning as an electrical discharge between two points. In a thunderstorm, positive charges build up in the top part of the cloud and negative charges build up in the bottom of the cloud. There are positive charges that reside in the ground, so when enough differing charge builds up between the bottom of the cloud and the ground, you get an electrical discharge or lightning. This particular lightning strike is called a negative strike. On the flip side, a positive strike occurs when there is enough negative charges built up in the ground and positive charges built up in the top of the cloud. Usually air is an insulator, but with enough potential difference to travel a particular difference, the air can be turned into a conductor.