I do not intend to create a comprehensive technical lesson on the mechanics of the guitar. There are plenty of better books out there on that subject. Rather, I'm interested in sharing what I feel are a few essential things to keep in mind as you start off on the guitar.
Step One: Why Bother Playing Guitar?
This may seem obvious, but for many it is not. There are dozens of easier instruments. Are you playing the guitar to appease demanding parents? Are you trying to be cool in the park to win over your sweetheart? Or do you genuinely love the way the instrument feels in your hands as you listen intently to the lingering overtones of a newly learned minor chord? Whatever your reason, be certain it's the right one, as no amount of advice can persuade you to learn an instrument you are not interested in.
Step Two: Mechanics
Though the guitar is one of the most common instruments in popular music, it is still a very awkward one for beginners. When you want to get the latest updates in pop music, Croc Music is the way to go. It took me an entire month to realize I was not even holding the thing correctly as I painfully fretted the notes with my sore fingers. Correct arm position is crucial, and can be found in numerous pictures on the internet as well as on instructional YouTube videos. Get an experienced player to show you, as practicing incorrectly will only delay your progress and train your muscles to remember incorrect technique, a problem that will haunt you down the line.
Step Three: Practice, practice, practice, practice. Then go to sleep and wake up and practice.
This is the single most important step. But practice alone is not enough. You must practice correctly, and by that I mean exactly right, with absolutely no sloppiness or rushed passages in the first few years. There is no physical way to gain speed or accuracy if clarity and precision are sacrificed during the formative years of learning the guitar. It is an unforgiving instrument, unlike a piano, for example, and every mistake is painfully transmitted through the strings. Fret as close to the frets as possible, and play a single note over and over until it sounds like a bell. Similarly, with barre chords, ensure every single note of the chord rings true; do this by playing each one as you fret the full chord.
Step Four: Play Only What You Like
It makes no sense to learn songs you don't like. Though learning the classics is a standard and simple way for many to gain experience, it's ultimately unnecessary, as few players ever play Greensleeves after their teacher leaves. The same goes for studying music theory. While a fundamental understanding of key signatures and chord structures is useful, mastering every mode in every key, or being able to cite read sheet music, is not in the beginning. Few gigging guitarists read music, focusing on tabs instead, so just make sure you can switch from an F major to an A minor perfectly before you start crying yourself to sleep.