Usually at some point in the first month of lessons, I will teach all of my students what is called "relative tuning". That is to say, tuning so that the guitar will sound good by itself, without any other instruments playing with it. If you want to play with other musicians, you'll need to tune to "concert pitch", which is a standard tuning which all instruments follow in order to play together, in harmony.
If a guitarist is alone in the woods, can he still play his instrument "in tune" and sound great?
The essence of relative tuning is that each string on the guitar is tuned to properly relate, intervallically, to each other. In music theory terms, the interval between each neighboring string, with the exception of G to B, is a Perfect 4th. String G to B is a Major 3rd.
It's a lot easier to just show you what I mean (as opposed to trying to explain it in words, on a blog). So I'm just going to post my worksheet and let you figure the rest out on your own! If you have any questions or comments, please feel free to leave those below.
Don't forget, here's the link to my... post series on guitar lessons curriculum.