Everthing ND said plus:
I use a Control Wizard bought from nehydro.com, about $50 (I have CW ph meter which I could never calibrate so I got a better one with auto calibration). Are you in soil or hydro?
Big difference how you use them. In hydro just fill a shotglass with water and stick it in. Make sure you know the ppm of the water you're starting with. In soil you just test the water for ppm before you add it . For ph you need to soak the soil and test runoff. Either way you will need a calibration liquid, usually 1382 ppm, and ph 4 and ph7 for a ph meter.
Feel free to ask more questions, someone will answer.