What should I do to make sure I don't catch a cold from people I share apt with?

Handwashing. Forget the supplements, vitamins and other otc things that claim to "protect" against the cold - the best data we have is with handwashing. The alcohol gels work great for most things, even better than soap and water. Also, avoid touching your mouth, eyes or nose - that's how most viruses get in our system. Good luck and stay healthy!
Wash. Wash your hands often, do not rub your eyes and mouth with unclean hands. Ask your roommates to wash their hands when they are ill. Stay well rested and eat healthy. Many times you cannot avoid getting sick though.