Water makes up most of your beer and is a very critical ingredient. When you start, most tap water will make drinkable beer, if you know a few basics.
#1, get a water report. Many labs provide reports for brewing, Ward Labs is one we have used for years. Go get one right now, we will wait here until you get back as we really can't talk much before you do that..... waiting.......
OK, now that you know what is in your Dihydrogen Monoxide (H20) besides the H and the double 0, we can talk...
Most tap water can make decent beer. The great beer styles of the world came about due to the location of the people making the beer and growing the ingredients. They optimized what they made according to what they had. We have a modern advantage that we now have water profiles, and can add things or dilute to match any location around the world. The combination of malts with the water is what changes the whole game. Water does not know what dark or light, or stout or pilsner is. All it knows is that there are combinations of minerals and chemicals and acids and bases that buffer and react in a big soup called beer. Softer water is generally an advantage for lighter beers, harder water for darker beers. If you used tap water in Tampa Bay to try and make a classic Bavarian Pils, you might not get world-class perfect results, but I assure you that you'd still make good beer. Just as if you wanted to make a big IPA that really had incredible hop flavor, you might wonder why you cannot hit the mark. It's all water chemistry. The good news, you don't need to learn much about the water to get started making great beers that your friends will enjoy and that can win medals. Many competitions are won with tap water and extract kits with less than perfect processes.... so keep reading.
I will keep it simple using my local water as an example. Our water is good for beers that are amber or darker due to the water coming from an aquifer that is mainly limestone and having a harder than normal balance. Why is the color a factor? Well, the darker grains used for darker beer contribute to acidify the mash, and add balance. Your mash should have a PH of around 5.2 generally, and without going to much into that, it's all about that interaction of the grain and water to get a resulting PH in your kettle that works well.
The big two problem causers are Chlorine, and Chloramine. If your water has either one, you need to remove them, period. I don't know how many times I have had to tell even some of the smartest beginners, and even homebrew shops, that Chloramines will ruin your beer and relegate it to the "tastes like homebrew" category. Chlorine and chloramine can contribute to the formation of chlorophenols and give your beer a plastic or "band-aid" flavor. Even a slight bit of this is obvious to many people, and is a big problem with homebrewing as people either do not learn enough about their water before starting, or they do not listen when people try to tell them to treat their water.
Local homebrew shops can be a big root cause of the problem as they are the contact for new brewers, and some of them refuse to listen, learn, and embrace the proper water handling processes. These are the people, that for a few bucks, can sell you what you need to make better beer, and even include it in their "starter kits" but many times fail to do this. They could make a few bucks and improve your beer, but fail. Not all shops are like this, but it's a shame that we can't get them all to hit the most important basic things, especially when you ask them to do this and try to teach them about water. Frustrating.
Chlorine is simple, it will evaporate or volatilize if the water is left uncovered a few days, or if a simple filter is used. Simple filters can be a RV filter from Wal-Mart for $15, or most any at the local Home Depot, just read the package. Faster flow = more $$, it's that simple. Chlorine can also be removed almost instantly by a small addition of Potassium Metabisulphite (Campden Tablets). More on this when we deal with Chloramines.
If you have Chloramines, and your water system administrator will tell you that, you need to be a bit more careful. What is a chloramine and why is it harder to deal with? Chloramines are a combination of chlorine and ammonia, bonded together, and the purpose is so that the sanitation properties of the compound will travel all the way from the water treatment plant to your tap and remain effective the entire time, where basic chlorine will dissipate and become ineffective. How do we get rid of the chloramine compound??? Simple, mostly the same as chlorine except the chloramine will not dissipate on its own. Filter systems would have to be really good to assure that they remove chloramines as it needs a chemical reaction (read lots of carbon and more $$). To remove chloramine with a filter system you need long contact times with carbon to break down the compound. If you have no way of knowing what the flow rate across the carbon needs to be, and have a way of properly regulating it consistently, and determining that in fact you are removing the chloramines, then just treat your water as if it has chloramines intact. Better to be safe as it costs very little to do the right thing.
So, you can buy a great filter, but when I know there are chloramines I simply also treat with potassium metabisulphite (Campden Tablets). All it takes is 1/4 of a standard Campden tablet for each 5 gallons as a single tablet will treat 20 gallons.
DO NOT FOLLOW DIRECTIONS USED FOR WINEMAKING, THAT IS TOO MUCH!!!
You are facilitating the breaking of a bond, not killing yeast as the wine folks are doing. Big difference, and while a bit of extra potassium metabisulphite is not harmful, it can carry through the process and affect your yeast and maybe the taste of the beer. Keep it simple and do not overthink this, just do it.
OK, so now you know enough to be dangerous the the next homebrew club meeting or bottle share.
For my process I first filter the water through a particulate filter and then twin carbon block filters. This result is pretty good brewing water but the water here is still a bit "hard." I also have a reverse osmosis (R.O.) filter to produce water to dilute the local water. The RO water is close to distilled (only H2 and only O) with 11ppm. This allows me to break down the local water to match any other world location with sometimes some additional minerals and such. Once you learn water profiling there are many tools out there to help you shape your water.
Water: The Book