Wind starts to rub the surface of the ground and the buildings. Then, the molecules take electrons from them and carry them to the sky. The ground becomes positively charged and the bottom of the cloud becomes negative and upper part becomes positive (Charging by induction). Also, cloud is mostly consisted of vaporized water and is a good conductor compared to air. As a result, the cloud starts to build up charges (Charging by conduction).

The cloud has high potential than the surface of the ground. When the charges are strong enough to flow through the air (the insulator), then it takes the shortest course in the air that has a very low electric potential energy (See Chapter 13 - Section 1. Flow of Charge).

Most of the time, lightning hits the ground. However, because lightning flows from high potential energy to low potential energy, it can also flow to the other part of cloud if it has lower potential energy than the air.

When you play golf and you feel your hair is stood up, then it is better to take out all metal things such as glasses, watch, ring etc. to avoid lighting. Then, be in the lowest place and do not go near any trees. When lightning hits trees, it is highly possible that the current flows through tree and it will spread on the ground horizontally.