With a tuner, you just need to remember the notes of standard (or other) tuning (as Kole posted.) Pluck the thick string (6th or E string,) now look at the tuner and tune your string until your tuner says E. Tuners will have a digital or analog readout of how close you are to a perfect E or A or any other note.



The pic is kinda small but on the left is a Needle type tuner, and on the right is a led type tuner. The top left pic is a not in tune E, the one below that is a perfectly tuned E. On the right the first pic is out of tune E, and next to that is a perfectly tuned E. Hope that helps.