Error 99 on that body can be caused by many different things including a weak battery. Other than to clean the battery terminals inside the cavity for the battery & the lens terminal set on the lens & in the body there isn't much you can do.
http://www.learn.usa.canon.com/resources/articles/2012/eos_error_msgs.shtml
"A skill is developed through constant practice with a passion to improve, not bought."
