Water chemistry for brewing is a complex topic, which is why entire books have been written about water and brewing. Unfortunately, many of these books assume you have a good knowledge of chemistry, along with microbiology and brewing.
Assuming you have access to a “reasonably good” water source, you should not have to do much to your water to use it to brew beer. Most municipal water in the United States can be used right out of the tap with no adjustment. If, however, you have a well or other source with particularly hard or soft water, then you can purchase bottled water from your grocery store and use that instead.
Interestingly, extract brewers can use distilled water for their brewing because all the minerals needed for brewing are already in the malt extract. When the maltster created your malt extract he/she mashed grains and then concentrated the wort down by removing the water, which means the ideal set of ions for brewing are already in the concentrated malt.