Qualifications
You need to be able to carry a tune. In fact, you should be able to bust out into ear piercing high note full throttle at the drop of a hat. Rock singers are not timid.
Speaking of, you can't be afraid of crowds. You must embrace the mayhem.
You can be shy in private, but once you get on stage, you need to own the joint.
It also helps to have thick skin. You'll need to withstand harsh criticism and boos, no matter how popular you get.