Members in my club have said Milwaukee tap water is good brewing water and they don't add much if anything at all. I started adjusting as soon as I started all-grain brewing because I can and I was interested in having another variable I could control if I wanted. I don't want to bog you down with too many details, because most of it depends on what you find out for yourself, but for starters:
1) I use 1/4 Campden tablet per 3-5 gallons of water. I pop them in when heating up for initial strike water or sparge to dissolve. Leave the lid off and stir occasionally when heating up. This helps remove the chloramine that the city puts in its water. Have I noticed a difference if I've forgotten? No, too many other variables. Have I done a side-by-side? No. I add it for insurance sake.
2) For your first AG, a beer that is 11-15 SRM color units (amber to slightly red) and low-to-moderate bitterness will require no other major water additions for your mash. I could be wrong, but I've noticed some moderately bitter (30-40 ibu) beers have needed extra conditioning for the slightly odd/harsher bitterness to mellow out. This might be a result of our moderate water hardness (bicarbonates, that you'd need to remove or dilute) or something else related to my system.
3) If you want to brighten/sharpen the hops, use a teaspoon or so of gyspum in the boil. If you want more rounded flavor or highlight more of the malt character, use a pinch or two of non-iodized salt (Kosher) or a teaspoon of calcium chloride. If you're afraid to add any of these in the boil, you can add them to a glass of finished beer to see what they do.
I'd keep it simple for your first attempt, and if you have a favorite extract recipe, begin with that in all-grain. Let us know a recipe and I can be more specific if you'd like. Good luck!
