Getting the guitar in tune is paramount to making the guitar sound like you're producing music. I tend to be overly sensitive about the subject and find myself checking to make sure that I'm in tune more than I need to. (My wife thinks that I tune so often that it's become another hobby of mine.)
As you may recall from the diagram in Lesson 1, the open strings on the guitar are E-A-D-G-B-E, where the numbering of the strings will be 6-5-4-3-2-1 respectively. When used in a C Major scale, they look like the illustration below...
Using a tuner is by far my favorite way to tune. There is a wide array of tuners available on the market, and most that you would find in a music store will fit nicely in the case with your guitar. Generally, look for a tuner that is easy to use. It might help to ask other guitar players to see what models they prefer. The following illustration shows a typical hand-held tuner, where a small needle moves back and forth depending on whether the string is sharp (too tight) or flat (too loose).
Okay, I admit - not everyone has access to a piano; they tend to take up a lot of space if you don't really play one. But if you do, it's a great way to get a guitar in tune. (Provided the piano is in tune, of course.) The illustration below shows where to find the notes that correspond to the strings on the guitar...
There may be times where you are working with other musicians, and you can tune instruments with each other by matching pitch, although sometimes that may involve different octaves, as when tuning a bass to a guitar.
Whenever possible, all instruments should use the same reference - such as a digital tuner. I was once playing guitar in a musical setting where one guitarist kept tuning her guitar by ear using one or two specific chords. This is always a bad approach to tuning a guitar anyway, but since the other musicians were using digital tuners that meant that she would never be in tune with the rest of the group.