Sit up straight and breathe from your diaphragm. When breathing correctly, you will pull air into your stomach and feel your stomach and sides expand. Most people can easily expand the stomach, but must focus to learn to expand the sides.
Form an embouchure by tightening the muscles around your lips. Pretend you are sucking through a straw.
Keeping your lips taut and squeezed together, blow through the center of your lips. Avoid letting any air escape through the sides or corner of your mouth. With practice you will be able to create a proper buzz for trumpet playing.
Increase the tension in your lips to vibrate your lips faster and create higher pitches. Loosen the tension in your lips to reduce the tension creating lower pitches.