couple questions about ph testing and all that stuff

Sorry if this has been asked before but im trying my best to learn as much as possible about this. Here is my setup and questions.

I have a ufo 90w led light right now (prob go to something else). Got 2 plants right now, only 1 week old. here are my questions:

I bought a ph tester/water moisture/light tester. I am not 100% sure how accurate it is but i was wondering do i need this? http://www.amazon.com/pH-Control-Kit-General-Hydroponics/dp/B000BNKWZY/ref=pd_sim_ol_2

Do i need to test the ph level of the soil and the ph level of the water i use to water the plants? Im a little confused.

I f you can get your hands on it. They have a little thing that looks like a meat thermometer and you stick it in the soil to test the ph.

it works pretty well but its always best to have a digital one..its worth the money..

i use those drops and the digital to make sure my digital is close and doesnt need calibrating..you kinda get the hang of it after awhile

Should I be testing the ph of the water and soil or only water. Or only soil? I have a tester for ph for soil. Dunno how accurate it is.