when i first started out i used tap water, some people get away with that but depending on where you live tap has different crap in it so its best to go to your local fish store and buy water from them.
i got two five gal jugs for 3 bucks from walmart and my lfs fills them for 90cents a gal for saltwater and 30 cents a gal for freshwater
best part about all this is. you know its good water(unless your fish store sucks, ha) and you dont have to mix it...or you can go buy a reverse osmosis unit (not sure on price) and make your own from tap water.