It depends on how complicated you want your brewing to be.
Generally, if your tap water is good enough to drink, it is good enough to brew with.
You want to be sure to remove any chlorine or chloramines, by letting the water sit out overnight (or boiling) or dosing with campden tabs, respectively. If you use distilled water, you will need to add some minerals, check out the brewing water primer
in the brew science forum for a ton of mostly useful info.
Spring water is useful. You can usually find the mineral content of most commercial spring water online. It is very common to dilute tap or spring water with distilled water in order to reduce some mineral properties of the water for desired styles.
Adjusting your brewing water takes a fair amount of research, usually a commercial water analysis (wardlab.com
), minerals from your LHBS, a fancy spreadsheet like bru'n water
or brewers friend
, some trial and error, and a lot of time.
There are loads of very knowledgeable people on here, so ask away.
Also, the Kaiser
have excellent websites to check out.