There are several threads about this here and there, but I will summarize briefly to get you started.
Diluting hard water with pure water is the safest way to lower the hardness. The KH (carbonate hardness or Alkalinity) will also lower, and the pH will usually lower too. "Pure" water means distilled, reverse osmosis (RO) or rainwater if this is easy to collect and otherwise safe. You can buy the first two, or prepare your own.
The GH/KH will be reduced proportionally. For example, if you mix half tap water and half pure water the GH/KH will be half what it was. The pH may or may not lower much, depending upon the KH which buffers pH, preventing fluctuations. The higher the KH, the stronger the buffering capability.
Initially doing this to fill the tank is straight forward. It is the prepared water for water changes that takes some effort, as you need to have some means of preparing the water outside the tank before adding it to the tank once it is mixed.
If soft water fish species are the intention, you could use basically "pure" water. A GH and KH basically zero is what I have out of the tap, and I add nothing (except fish foods, and very little liquid plant fertilizer) to increase mineral content. My point is that softy water species do not need mineral other than what they get in foods; this is how they live in their habitat. Some species could manage with some GH, it depends. Not all soft water species occur in this type of water.
