Assuming you are holding the instrument correctly, place your lips to the mouth piece. You need to vibrate your lips together, or 'buzz' as brass players call this technique. The vibrations will continue through the instrument and out of the sound hole. To change note, the valves are depressed with the fingers. Different combinations will give different notes. To change pitch, and thus open up a greater range of notes, the lips can be vibrated more tightly or loosely, which will higher or lower pitch respectively.