There's an abundance of guitar information out there on the web, some good, some not. I stumbled across Justin Sandercoe's site a year ago and now tell everyone about it. The lessons are conveyed so clearly, concisely and in the most congenial way. The site is laid out logically as well so you can to go straight to your area of interest... beginner, blues, rock, folk, jazz, rhythm, fingerpicking... it's all there and more. Spend ten minutes with Justin and you'll not only play better but feel better too. From novice to know-it-all, everyone will learn something from Sandercoe.
A six-string guitar has five musical-intervals between its consecutive strings. In standard tuning, the intervals are four perfect-fourths and one major-third, the comparatively irregular interval for the (G,B) pair. Consequently, standard tuning requires four chord-shapes for the major chords. There are separate chord-forms for chords having their root note on the third, fourth, fifth, and sixth strings. Of course, a beginner learns guitar by learning notes and chords, and irregularities make learning the guitar difficult—even more difficult than learning the formation of plural nouns in German, according to Gary Marcus. Nonetheless, most beginners use standard tuning.
Unlike many string instruments which use a bow to create a sound, guitars are played by plucking or strumming the strings on the instrument, which vibrate to create a sound. Chords are played when the guitarist holds their fingers down on more than one individual string to change its length, which in turn changes the frequency of the vibration created - in other words, it creates a different note. Performers often use guitar tabs, rather than sheet music, when learning music: this acts as a kind of 'map' to teach guitarists where to put their fingers on the strings.