Don't use tap water.
How often depends on your tank and your bioload. For a year I just had two stars, I did a water change about every three months...but I also have 100lbs lr.
Some people do some weekly, bi-monthly, or monthly. You'll have to find what's right for you tank.
if you have cc you'll need to clean it, they sell the tubes at walmart.
I mix the salt right in the gallon jugs, let it sit over night, then I add it, after I check the salinity. 5 gallons would be easier, but I don't have a place for that, plus, how the heck would I get the water from the 5 gallon jug to the tank?
refill water from walmart is .33 a gallon