Changing aquarium water?

How often should I change the water in my aquarium? And should I just change part of it? I also have a gravel vacuum, so how often should I be using that? I probably should have done some research before buying all this stuff, lol. If it makes any difference, it's a 10 gallon fresh water aquarium, and we have six fish--three Glofish and three other ones. They're some type of tetra--they're peach colored with a little black on them. If I do need to do a partial water change, how do I go about conditioning the water? When I set it up, I used a water conditioner for a 10 gallon tank. If I add fresh tap water, should I add a little more conditioner?