Most brewers don't worry about the fine points of water treatment until they become somewhat advanced. However, those who do venture into this area often find the rewards significant.
Assuming that your water tastes reasonably pleasant, you can normally use it in making beer. Obviously, such problems as a high iron or salt content might require getting water from an alternate source, but in most situations, your water is probably okay. Different types of water supplies, however, will affect your results, either positively or negatively, depending on the type of beer you're trying to make. Often the reason a particular city became renowned for a certain type of beer was because the water supply was especially suitable.
If you're starting with de-ionized or distilled water in your brewing, this table gives you some guidelines for treating the water to make ten gallons of several types of beers. If using tap water, as most of us do, you should test your water, or have it tested, and adjust these amounts accordingly. Note that if you're working with harder water than is appropriate for the style you've selected to brew, you may need to adjust with a percentage of de-ionized, or distilled, water.
If you're on a municipal water system, getting the statistics on your water is relatively easy. You can usually call your water company and get figures on both the permanent (sulfate) and temporary (carbonate) hardness, as well as the salinity (salt content) of your water. If you're working with well water, you can get approximate hardness figures with a water hardness test kit. Figures in the following tables are expressed in parts per million (ppm.).