Water
This is far more important than most people imagine. Indeed, breweries are famous because of the water supply they have (or did have before pollution ruined it), and upon the type of water supplied to your district depends to some extent the quality of your beer.
You may be quite satisfied with the beers you make regardless of the type of water in your area. However, to harden soft water and to soften hard water is quite a simple matter. Hard water is best for pale and bitter beers while soft water is better for mild ales and stouts.
If you are doubtful about the sort of water you have coming through your tap your local water board office will tell you whether it is hard or soft. You may then alter it to suit whichever type of beer you propose to make. Hard water may be softened by boiling before starting the brewing. That it will be boiled again during the process should be disregarded. A water softener has its advantages, but also its expense. Home brewing supply firms supply ‘Burtonising’ salts for treating all types of water very cheaply indeed and these do make a vast difference to the finished product, for they bring out the full flavor of the malt and hops. All water in the commercial brewery is treated in this fashion - hence their use of the word ‘liquor’ - brewery liquor, instead of water. Use the salts as directed by the supplier.
Do not imagine that you will be adulterating your water supply, you will merely be making good deficiencies; and, in any case, following the commercial brewer as closely as you can.