Modifying your water profile is one of the last hurdles for an advanced homebrewer. Changing your water will not make a bad beer good. It will make a great beer...fantastic.
Your city's water profile will affect your final beer. You can manipulate how a beer is perceived by fine tuning your water's mineral profile. For example: higher calcium and sulfate levels can make your hops and bitterness brighter and less will lend itself to a maltier beer.
To help me figure out the amount by weight of my water additions I use EZWaterCalculator. There's also the popular Bru'n Water (more powerful but has a big learning curve). Having tried both, I've found the numbers come out very close.
|City of Toronto|
|Ward Labs 2014
3rd Party Analysis
|Calcium (Ca ppm)||34.1||41|
|Magnesium (Mg ppm)||8.78||11|
|Chloride (Cl ppm)||26.5||27|
|Sulfate (SO4 ppm)||28||11|
|Total Dissolved Solids (TDS)||?||202|
|Total Alkalinity (CaCO3)||87.8||93|