You'll need a GH, KH, and pH test kit.
Test your tap water first to see where you're starting at. Add salts/baking soda to your spare tank with filter (a heater also helps) and let it mix for a few hours if not for a day. Test to see where you're at and write down your results so you know how much to use the next time you mix. It may take a couple of tries before you find just the right formula.
What kind of fish are you trying to keep?