How can I take care of my goldfish?

I'm completely 100% new at fish care. I kind of got stuck with this goldfish. No one else can take this fish and I don't want to just... flush him. So I kept him. I have a small tank with no filtration :S I don't have enough space for one. So I figure I'll just have to change his water more often. Which I'm okay with. But how often should I change the water? And can it be bad for him to do complete water changes often? :S:S And what about water conditioner? Help me please fish people!