Listen to a diverse selection of hard rock songs and identify what makes each voice potent and distinctive.
Practice proper breathing by inhaling through your nose while expanding your diaphragm (stomach).
Determine your vocal range by using a professional singing program such as Brett Manning's audio course. Are you a soprano, alto, tenor or baritone? You could also try singing along to a musical instrument to determine your vocal range.
Warm up before singing to avoid damage to your vocal cords. Warm-ups are basic vocal exercises like scales that are vital before a practice session. You can also try making varied noises as a vocal exercise. Some will be low, like a cat growling. Others will sound more nasal or grating.
Find hard rock songs within your vocal range that are comfortable for you to sing and emulate, and try them out in your own style.
When going for the high or hard notes, bend over at your waist to ease your voice into reaching hard rock singer potential.