You can use a keyboard or buy a tuner. The way that I do it is by using my keyboard. I pluck my open E string (4th string) and I tune that to the keyboard. Once my E string is tuned, I then go to the closed A note, (which is the 5th fret on the E string) and pluck that note and tune my A string according to closed A note on the E string. Once my A string is tuned, I go to the closed D note on the A string, (which is the 5th fret on the A string.) pluck that note and tune my D string according to the closed D note on my A string. Repeat this process until you tune the first string which is the G string.
Hope this helps.