The first turn you learn, and the last resort for skiers of all levels when complete control is vital. You can use it to stop as well as turning. Also called wedge and pizza. It is performed by pushing the tails of the skis out while leaving the tips close together. This makes a wedge shape (or, for kids, the shape of a slice of pizza). To ensure that the edges bite into the snow (this is how the thing works) you should keep your knees close together. The turn is performed by putting (or shifting) more of your weight (standing harder or more) on the ski opposite to the direction in which you want to turn. You will find that you will turn until your skis are across the fall line.
The snowplough is quite a tiring way to turn and control your speed. It is important to learn the snowplough, but the faster you progress beyond it, the more fun you will have. Advanced turns use combinations of gravity, location of your centre of gravity, edge control and the rebound of the skis to assist turning. With the snowplough turn you are fighting these forces. Professional lessons are the fastest way to achieve the progression.