How do you ph your water?

How do you guys ph your water? I was gunna buy the general hydroponics ph down. I was wondering tho if I could just use lemon juice cus I've heard that's a way to bring ph down. I was planning on using distilled water because I have very hard tap water that usually kills plants. Should I use the tap water anyway and just leave it out for 24 hours then ph it? When you leave out the water do you leave it uncovered, I'm assuming so. Thank you for reading any advice is welcome I appreciate it I haven't started growing yet but I'm going to start soon once this is figured out!

Thanks everyone so much!!!! I appreciate everyone's advice I've read everything you all have said I appreciate the time you guys took to respond!!!!