Close your mouth and blow out through your lips, making a buzzing sound as you do so. Hold the mouthpiece against your lips so that the center area of your lips is against the cup of the mouthpiece.
Take a deep breath from your diaphragm and blow into the trumpet, making the buzzing sound with your lips at the same time. You may find that it is possible to make several different notes. Practice playing the lowest note, which is a C note.