There are a couple of reasons to make changes to your water mineral profile.
The first is to accentuate the flavors of your beer. Higher Chloride levels bring out the malt flavors. Higher Sulfate levels give a drier finish to your beer which accents the hop bitterness.
The second is to provide for the needed minerals which aid in yeast health and flocculation. Calcium and Magnesium both are used by the yeast in their reproduction cycle and during the flocculation and settling stages of fermentation.
The third is to help with controlling the pH of your mash to optimize the enzyme activity and prevent harsher compounds, like tannins, from being released. The choice of mineral salts used for the first and second reasons above will help with adjustment of your mash pH.
I would recommend first searching for and reading some of Brad's blog posts about water and mash pH along with some of the interviews specifically about water quality with John Palmer and others. Next, download a copy of Martin Brungard's treatise on water at https://sites.google.com/site/brunwater/water-knowledge. His spreadsheet is designed much the same as the water tools in BeerSmith. The nice thing about BeerSmith is that it is linked into your recipe and so uses those inputs to balance your water to match the profile you want.
As part of this, you will need to get an analysis of your water source. Many people use RO filtered water which removes the majority of the ions so that you are starting from scratch. If you are on municipal water, your water treatment department may or may not have a good report on the chemical make up of your water. Most of the ions you need concentrations for are not part of their standard test screening. As an alternative, you can get your water tested at Ward's Labs (they have a specific water test for brewers) or use a water testing kit such as BrewLab (http://www.lamotte.com/en/food-beverage/brewlab) to test your water yourself. This is a more costly option, but if your water changes seasonally, it is well worth it.
If you are looking to control the mash pH, you will want/need to invest in a pH meter. There are test strips available as well which are of lesser accuracy but work fine. There are a number of affordable pH meters on the market for water testing. My recommendation is not to get the very cheap ones and stay away from those without automatic calibration.