Turn the digital tuner on and set it to the mode that allows you to identify pitches. Generally, this is the default mode and is listed as "Main" or "Tune" on most tuners.
Pluck the guitar string as you would normally play it. Hold the pick between your thumb and index finger and use the tip to flick the string.
Identify the number on the readout of your digital display. It will be a three or four digit number and a letter name. This number tells you how many times per second the string is vibrating and is also your frequency in hertz. The letter tells you the name of the pitch. For instance, if you play an A that is perfectly in tune, it will read "440hz A."