Trees are often the tallest objects around and contain many pointy tips. The electric field is strongest near the pointed tip of an electrical conductor, and so trees often make for great targets for lightning strikes. Trees don't always conduct electricity as well as you do, since people are made mostly out of saltwater. The current from a lightning strike may leave the tree and jump over to you and follow your body on its way to the ground. Standing out in a field far from trees may also pose risks too. In this case, you are the tallest object around and may be a target for lightning strikes. If you are carrying metal , it could raise your risk. The safest place to be during a thunderstorm is a building or a car. Electricity can flow through the metal sides of a car and not through you. Lying down on the ground is also not  a bad idea, except that during thunderstorms it is probably wet down on the ground. It's best to seek a house or car to stay in during the storm.