I'm 21 and I want to be healthy. Will I get hanging skin if I lose 30 pounds?

Typically not. Usually young skin handles weight loss well. If your skin has a lot of stretch marks then in those areas the skin can appear more lax. But getting healthy should be your priority, and new lasers can tighten any loose skin you may have after the weight loss.
No. Weight loss of 30 lb in your age group should not result in hanging skin.