Hardness (GH) and KH are not the same thing. Most water companies, if they give it at all (not many do), call KH alkalinity.
GH is a measure of the amount of calcium and magnesium in the water. This is what fish profiles mean when they say a fish needs hard or soft water. Fish have evolved to cope with a certain level and many will not thrive if kept in water of the 'wrong' hardness.
KH is a measure of carbonate and bicarbonate in the water. Fish keepers like to know it as it is a measure of the amount of buffer in the water; that means how stable the pH is likely to be. With a low KH it is very easy to alter the pH, but with a high KH is it very hard to alter it.
As Steve says, getting the shop to test the KH of your tap water is the best way to start. The critical value is 4 degrees or 70ppm, whichever unit they use. If it is lower than this, get a KH tester and check your pH every time you do the other tests. Up to 6 or 7, don't bother with a tester but check the pH frequently. If it's over 10, you won't have any problems.
But if it is less than 3, you need to take action, I'll tell you what if you find it is that low.
There are 2 concerns with low KH. This is a measure of the amount of carbonate and bicarbonate in the water. The filter bacteria need some in the water to grow, and they use it up as they do so. And both nitrite and nitrate, made by the cycle, are acidic and will react with carbonates, reducing the amount in the water. Once the amount of carbonate has dropped sufficiently low the acidic chemicals then have nothing to stop them dropping the pH - and the bacteria won't multiply below 6.5. And they won't grow anyway if there isn't enough carbonate.
Some areas of the country have a KH well over 10. Here, there is no danger of a pH crash during cycling. However, if you do have lowish KH, there are ways to boost the carbonate during cycling, either water changes to replenish the carbonate or by adding bicarbonate of soda (from the home baking section of the supermarket), I can tell you the dosage. Bicarb is fine to use during cycling but not when you have fish as all the sodium (the soda part) is as bad for fish as it is for us.