The guitar is the instrument of modern music, the spirit animal of youth. It is synonymous with cool independence and fresh creativity. Whether it be a battered acoustic or a thrash metal axe, it is imbued with the same core romanticism: revolution, newness and change.
Because of this, playing the guitar is a deeply rewarding activity. It has social benefits that no other instrument can provide, and an immediacy of use that isn't possible with practically every other common instrument, including its rock brethren, the bass, drums and keyboard. You are able to pick up any guitar (and that means any) and play a few chords, feeling the familiar movement of your fingers over the fret and hear that twangy, compelling sound, often to the mirth of any assembled company. It is a sound that translates equally well to the camp fire as to the stage, and everything in between.
So what are you waiting for? You can pick up a secondhand acoustic for under £50 if you're lucky: new strings cost pittance, if they need replacing. To begin with, learn to strum. Invest in a pick (or, to give it its proper Latin name, a plectrum) and get the feel for hitting the strings in rhythm. Once you feel comfortable with that, learn some chords. The basic ones at first will be all you need: G, F, C, A, D, E (perhaps B). From that basis you have most pop music straight off the bat: you'll be singing along to Knockin' on Heaven's Door, Sweet Home Alabama most of The Beatles' first album before you know it. From there, learn how to form a minor chord from the major root, and how to create all the sharp/flat notes through bar chords. You'll then be singing Hallelujah, Wonderful Tonight and... well, anything really. The internet has the chord progressions for every song ever, including classical music! Even if they technically require slightly more advanced forms like suspended fourths or jazz chords, playing the base note for the time being will suffice.
From this base, you can learn to pick out individual notes to make licks and riffs. Now may also be good time to learn to make 'power' chords (sounds intimidating, actually very easy), if you're interested in pursuing the rock/metal avenue. There's nothing quite as satisfying than the first time playing your favourite riff, for real, instead of on Guitar Hero or Rock Band. And it's not as hard as it sounds. Famous guitar classics like Smoke on the Water can be learnt in minutes.
Now, the guitar is an instrument which takes a comparatively short while to become fairly competent at, but an incredibly long time (practice-wise) to fully master. If you've got a hankering to become the next Jimi Hendrix, Jimmy Page, David Gilmour, Eric Clapton or Carlos Santana, be prepared to dedicate a truly significant proportion of your time to the guitar. It should become your prime passion, as natural to you as breathing. Hours will be sunk into practicing on a daily basis. Your fingers should dance across the frets, knowing intuitively where to go without your mind becoming involved. To reach such heights of skill is no mean feat.
One can have help ascending the mountain, of course. Guitar lessons from pros, in the form of private tuition, can pay dividends. Learning the guitar isn't something that works well in a classroom environment: to be effective it requires your needs to be concentrated on fully.
Ultimately, the end result will be worth it. The guitar's great strength is its flexibility: whether you aim to take it to the stage with your band or record your own music alone, it will open up new cultural, mental and personal frontiers. It may even present you with a career path, if you're lucky.