Relax the muscles in your face and throat. Open your mouth and drop your jaw.
Hum a high pitch by imagining a woman saying "ooooh!"
Hold the high pitch for a moment and gradually slide your voice lower until you are imitating a man saying "oooooh." This sliding pitch exercise is often used as a vocal warm up.
Play a note on a piano or keyboard. Hum that note. Then, play a note that is four to eight notes to the right of the first note. This will be a higher pitch. Play a note that is four to eight notes to the left of the original note. This will be a lower pitch. Practice playing keys of the piano up and down step by step as you hum the notes that you hear. Keep in mind that most people will be capable of singing only 10 to 20 notes located near the center of the instrument.
Listen to other people sing or speak and try to imitate the pitches that you hear in their voices.