Find an instrument that feels comfortable to the touch, or your playing will sound awkward and strained. Do not settle for the first acoustic guitar that you spot in a music store or pawnshop. Avoid choosing a classical guitar as your introductory instrument, because its unique configuration--which includes three nylon strings--is better suited to advanced players with a higher level of technique. Also check carefully for critical defects that can stop enthusiasm in its tracks, such as warped necks, buzzing frets, dead strings and loose tuning pegs.
Learn to tune your guitar, which can be done several ways. If you have access to a piano, strike each key that corresponds to the strings on your guitar, moving from low E, to A, D, G, B and high E. Pluck each of your guitar strings in turn, and then adjust the relevant tuning peg until the tones match. This can also be done by using a pitch pipe made for guitar or an electronic strobe tuner. Put a premium on telling different chords and pitches apart, which is crucial to playing well with other musicians.
Spend some time practicing basic strumming patterns. Rest your pick lightly against the bottom E string, and then strike it against the relevant strings for the chord you are playing. Notice that, unlike an electric guitar, it does not take a lot of power to produce a distinctive tone--which comes through the sound hole of your instrument. As the Who's Pete Townshend has shown in classics like "Magic Bus" or "Pinball Wizard," varying the frequency and intensity of basic strumming patterns can add moods that would otherwise leave the music sounding empty and flat.
Experiment with light, medium and heavy string gauges until you build up calluses on your fingers and get used to a type that works best for you. As a beginner, avoid heavy gauges--which come into play for specialized styles, like country-blues--until your fingers get accustomed to playing regularly.
Study books, charts or online guitar sites to learn basic chords and string positions. Nowadays, reading music is not a must for successful playing, because many songs are written in tablature, which focuses on finger positions instead of note values. Learn both methods, which will broaden your ability and make it easier to land gigs--such as musicals--that require the reading of music.
Learn how to change your strings, and do it regularly. Sticking too long with a particular set causes the strings to lose their brightness, resulting in a flatter, deader sound. Pay attention to the action, which is the distance between the strings and frets--which results in buzzing if left too low or becomes hard to fret chords if left too high.
After you meet enough musicians, seek recommendations for a good guitar teacher who can help gradually improve your playing. If possible, talk with the prospective teacher ahead of time to map out benchmarks to map your progress. If the rapport seems awkward or the teacher's priorities seem sharply at odds with your own, look elsewhere until you find a better fit.
Practice playing with a pick, as well as your fingers, so you learn how both ways feel, and become a more well-rounded guitarist. Fingerstyle playing is especially important in genres like blues, classical or folk music, although you may sacrifice some of the speed that only a pick can provide. For additional variety, arch your thumb over the guitar neck to play bass notes--a technique that the Kinks' Ray Davies uses in many of his band's classic songs.
After you feel more confident of your chording, try to add some basic fingerpicking patterns into your repertoire. Rest your right hand's thumb on the low E, or sixth string, leaving your index, middle and ring fingers on the first, second and third strings, or high E, B and G, respectively. Pluck the first note of the pattern with your thumb, following in quick succession with the other fingers. This will give your playing more variety beyond basic strumming.
Explore alternate tunings to broaden your creativity. Many songs, particularly from the grunge and alternative-rock eras, go against the grain of standard tuning, which follows the conventional string setup (low E, A, D, G, B and high E). You can achieve the same effect with a capo, which can be placed across the frets to change the pitch, thereby opening up new chord positions.
Take breaks if a particular song or musical passage does not improve with further practice. A 15- or 30-minute timeout can do wonders for concentration and focus, instead of beating the same thing to death. A relaxed and refocused mind is the best antidote to boredom or stagnation.