As mentioned above, visit the Water Primer first. Its geared toward building simple brewing water from RO or distilled water and is a good place to start. When you want to move on from that point, look at the information contained in Bru'n Water.
Depending on your water supply, you might find that it could be used for brewing. I recommend that if the unsoftened water still tastes good, you should consider revising the plumbing of your water softener so that it feeds only your water heater. That is really the only equipment that you really want to protect with a water softener.