Simply put, when I learned to brew a neighbor told me that our aquifer was way too high in iron; he said you have to use bottled water. I used gallon bottles from Menards for extract beers until I discovered how much minerals were already added to a base RO to make their product. I then went to all grain and used store made RO, unknowing of the TDS, but the flavor was better than the bottled that was sold in gallon containers at Menards. Then I jumped to an RO system at home with a TDS meter - 350 to 500 TDS in and 10-19 TDS out.
I weigh my CaCl2, gypsum, Brewtan B, and SMB using a small hop scale (reads to .001 gram, but likely only accurate to a tenth) and use a small syringe for my Phosphoric acid additions. All I can say is that starting with RO that low, I can pretend it is distilled for the most part. but I am still learning and consider myself a water neophyte.
My son is starting to brew (has yet to make his first batch) and my only advise was that he needed to buy RO or distilled water to use in his brews and he can adjust from there. He will likely make good extract beers with that starting point and with time, he will likely seek to adjust it for styles. But starting out, I didn't want to overwhelm him (he is 24 and has been my "bottler" since he was 12). I hope he enjoys the hobby as much as I do. I tell him frequently that my greatest regret in life was that I waited until so late in life to start homebrewing! (I was 46).