If your brewing water is too hard for the style you're trying to mimic, you'll have to work a little harder to correct it than if the water was too soft. Then draw the water from the vessel leaving the mineral deposits behind. If your brewing water is too soft for the intended beer style, you can add mineral salts to increase the water hardness to the desired level. Add gypsum to the water to increase its calcium sulfate content, Epsom salts to increase its magnesium sulfate content or table salt to increase its sodium chloride content. In addition to the relative hardness of your brewing water, you should be concerned with the its chlorine content.
Chlorine is added to city water supplies to control bacteria and living organisms.
Unfortunately, chlorine also adds off flavors to your beer. You can remove chlorine from your water either by letting the water stand in a pot over night, pre-boiling the water or running it through a charcoal filter. Using a charcoal filter is the recommended method.