I started guitar in my mid forties.

Four years later, I've reached the point where (some) non-musicians are envious, and actual musicians categorize my sounds as “music” more often than not.

There's lot of great advice on this thread. (Especially about deliberate practice; and about avoiding instruments so cheap they'll just increase the difficulty while making you sound worse than you are.) I'll fill in a few notes from my own journey. YMMV.

Choosing a guitar:

* Steel string is hell on the fingers for a beginner. It is effective altitude training for nylon and electric. I love the tone and I'm really glad I started with steel string. However, had I known how hard it would be, I don't know that I'd have advised myself to start with it.

* Electric is easy-peasy (callus- and finger-strength-wise). I think a decent electric is cheaper than a decent acoustic too. Con: (1) you're less likely to pick up the instrument for two minutes of spontaneous practice when you walk past it on its stand. (I've probably got a lot of practice in from two minutes that turned into fifteen that turned into fifty.) (2) If you're a unmusical nerd like me, it's easy to get distracted from the hard work of making cleaning notes, by the comfort zone of playing with electronics.

* Nylon (classical) is easier on the fingers than steel but more difficult than electric. It might represent a happy medium between beginner fingers and practice activation energy, even if you don't play classical. Beware: new nylon needs to be tuned again every five minutes. After a month, it finishes stretching out, and holds its tuning about as well as steel.

* You can play any style of music on any guitar, except: (1) you can't really bend (a blues staple technique) a nylon string; (2) electric guitars have much greater sustain than any acoustic guitar: if the music you want to play sounds more like a violin or voice (sustained, tonal) than like a piano (percussive), you need an electric; (3) let's not talk about slide.

* What everyone else says about too cheap a guitar (or, “guitar-shaped instruments”, such as acoustics available for <$150-200). Specifically, you're looking for (1) “low action” (don't have to press a string too far for it to reach the fret), (2) good intonation (fifth fret low E is the same pitch as the open A, etc.), (3) stays in tune. Depending on your tolerance, you may also require (4) good tone (timbre). Cheap instruments are generally deficient on all of these, and this can really kill your motivation.

Decide whether you want to strum (harmony) or play individual notes (melody, or “lead”; also most of classical). These are almost different instruments. (For the beginner. A lot of melody is playing off of chord shape hand posiitons; and, coming from the other direction, embellished chords get to sound a lot like lead.) If accompanying (you sing or play harmonica, or can play with in a band), strumming is a valuable contribution. If you're playing solo, you probably need to play lead in order to have music that can stand its own.

Decide whether you want to play with a pick or finger style. It's easier to be loud with a pick, and it's especially easy to be loud strumming with a pick. If you're playing acoustic with a band, this may matter to you. Classical, on the other hand, is always finger style. Otherwise, don't make too much of this decision – there's a fair bit of technique to each, but most of what you need learn as a beginner applies to both, and if you continue with guitar you'll probably want both under your belt anyway. Just find some musicians or music that you want to sound like, and do what they do.

Choosing a learning style. Try each of the following; see which one(s) seem effective for you.

* Learn from an instructor. If this is how you learn best and you have funds and access, find someone you can trust, and then ignore everything else I've written.

* Play by ear. Listen to something, try to make your guitar sound like that. Many of today's older guitarists wore out vinyl records this way. Man I wish I could learn this way.

* Jam with friends. This may work if you're musically talented (I'm not), and/or coming from another instrument. Ditto.

* Learn by watching. Watch people's fingers, do what they do. If this is your learning style, YouTube is the golden age. My pseudo-science theory is that if you're good at learning from watching (e.g. sports, dance, watching people's hand motions when they sew, cook, juggle, drive, etc.), maybe you have a well-functioning mirror neuron system and this will work well for you. Yet another style I aspire to but suck at.

* Play from tab. Learn to play tab, and get some pieces under your belt. I kind of feel like this is the least-musician-y thing to do, but it matches my strengths so much better than the others that I went with this. At this point the road forks: (1) Classical: now learn the (newer!) standard staff, and leave tab behind – that was your starting wheels. (2) Folk/pop/rock/etc.

* Play from standard notation. This is really only used in classical repertoire. Even for classical, tab (which is the older notation) is still easier as a stepping stone, and has the advantage of building in performance notes about fingering, but if you're working with an instructor or you're not going to be put off by learning several hard things at once, then skipping tab gets you to the endpoint more directly.

* Play from chord diagrams or lead sheets. This is good for strumming (harmony). I'm not a strummer; don't have much to say here.

Practice:

* “What you do is what you do every day.” 15 minutes / day beats two hours once a week

* “The journey is the reward.” Find a way to enjoy the practice itself.

* Allocate some time each day for (1) skills, (2) repertoire, (3) noodling around. When you're starting, you won't have any repertoire (2), and there may not be much distinction between (1) and (3) (“I'm just trying to get any single note to sound clean.)

* At any particular point, have one or more skills you're working on. I've iterated through: play without fret buzz, use each finger without fret buzz, finger adjacent strings, finger non-adjacent strings, chromatic scales, blues scales, various chords, various chord alternations, making a barre, hammer-ons and pull-offs with various fingers, flat picking at tempo, flat picking alternate strings at tempo, finger picking a chord, bending a note, banjo rolls. Spend fifteen minutes a day on each active skill. (Exception: when you're starting, you may only have strength or callus for five minutes. “It's a marathon not a sprint.”) I probably spent fifteen minutes a day on barre for a week or two before hearing anything that even vaguely resembled success; eventually, it locked in.

* Learn to play at higher tempo by (1) playing as fast as you can with zero errors. Drop the pace until you can do this. There's a theory that you can learn even faster by (2) playing even faster, with errors, so long as you also do (1). This has worked for me.

* Some great musicians swear by practicing with metronomes; some advocate doing without them so that you develop your own sense of rhythm. If the latter, record yourself and play it back against a metronome, so you can tell if your tempo is as good as you think is is.

* Isolate and repeat difficult sections. Spend a minute, or five, or fifteen, on the smallest problematic section (maybe a single chord, or chord alternation, or four notes from a melody); don't play the whole piece and only hit the problem spot every few minutes.