Not such a simple question Crusty but I'll try and keep the answer simple and then direct you to some more complicated references.
First it is the pH of the MASH rather than the water that you want to alter and you can do this with calcium salts (namely calcium sulphate and calcium choride). These will drop pH. Dark grains will drop pH. Acid will also drop pH - commonly used are lactic, phosphoric and citric.
Calcium salts will also contribute to flavour and other things - calcium is good for yeast health, efficiency, foam stability. Sulphate will help brighten hop profile but is generally avoided in dark beers and chloride will help push malt flavours.
Too much of anything is never good.
Try and start with soft water and build the mineral profile you want for the beer you are making. Add salts for flavour, assess the pH then adjust with acid if necessary. You should be able to get salts and food grade acids from good AG homebrew places like craftbrewer, grain and grape, MHB etc.
You can also get things like magnesium sulphate and calcium carbonate, neither of which I recommend. Calcium carbonate will raise pH and is only ever used in really dark beers due to the acidity of dark grains but its actual efficacy is dubious and there are other methods. Some still swear by it. I won't go into the other methods in this answer and it might be a suck it and see approach for you. Magnesium sulphate does nothing Calcium sulphate doesn't do already besides add magnesium and there is enough already in an all malt mash. Calcium sulphate does everything else and does it better.
I'll hunt up some lengthier links for you.
Easiest spreadsheet is the metric version of the EZ water calculator. More complex but probably giving a more accurate result is the brun water calc linked above.
Post 11 here has the article I wrote a while ago:
http://aussiehomebrewer.com/topic/46120-ahb-articles-water-chemistry/
Within that there are several references, including online references. Brun water knowledge page is also worth a look