I use RO water ph adjusted to 5.4-5.9 for hydro, it varies from week to week slightly during bloom...most tap water is 200 ppm/for soil you adjust water to 6.5ph...so you need:tds pen and ph pen (some company's sell a combo, I believe it also measures temp.
Now, you can use just tap water, but you need to adjust the ph and if you are growing in soil you can just use the water out of tap which is usually around 7 ph,
you just won't have as good of results, but back in the day many a grower smoked weed that was watered with plain old H2O...goodluck.