Hi and welcome to AA!
Great that you're doing research first, good for you!
There is a great test kit most of us use here, it's called the API Master Freshwater kit. It's a liquid kit that will test ammonia, nitrite, nitrate, and PH. Don't get test strips whatever you do; they are cheaper but not accurate. The API test kit is pretty widely available, most pet stores and fish stores should have it, or you could always order on Amazon.
Are you cycling the tank without fish or do you have fish already?
Take a look at the links in my signature below if you can see them and click on the one that pertains to your situation (new tank with fish or without). If you don't have fish yet I highly advise cycling the tank first fishless with something like pure ammonia (that guide will tell you everything you need to know). If you have any questions along the way we'll be happy to help. Good luck!