I first learned how to tune the guitar to itself - you know, using harmonics and/or simultaneously strumming 2 strings at the 5th (and 4th) frets. That's great, if you are jamming by yourself without any backing whatsoever. Over the years, I have practiced enough be able to "sing" the rough equivalent of a high E,,,but I'm old school, and I don't live alone a deserted island.
In a live situation on a pedal board, nothing beats a BOSS TU-2, a TU-3, or a TC Polytune. On the headstock, I really like the Snark products (I have a couple of SN-2's).
For everyday practice, here's the name of a free app that I absolutely LOVE: PitchLab Lite (In fact, I even prefer this version over the PitchLab Pro version, which I purchased, then uninstalled.)
Whether tuning to Standard Tuning (Concert Pitch), or any of the myriad of Alternate Tunings, the important thing is that I make sure to tune up BEFORE I start practicing. In this day and age, it makes no sense to mess around with an instrument that is not in tune - there is just no excuse for it, and only does harm in the long run.