When starting out, it's extremely easy to pick up bad habits, and much harder to break them. The most obvious would be to only strum downwards with the plectrum. I imagine most youtube beginner tutorials will advise against something as obvious as that, and there are plenty of self-taught guitarists out there who are exceptional musicians and highly competent, but getting lessons is a good idea if you can afford it and can find one you click with reasonably well. Most importantly, any half-decent teacher will pick up on what you're doing wrong, and correct you on the spot, which a youtube video can't do. They'll also set you "homework" of some kind, which forces you to pick it up and actually play it, playing things you probably wouldn't have otherwise bothered with but which nevertheless teach important concepts/techniques, and you might find you like some of that music.
Also, lessons aren't cheap, and if you're paying with your own cash there's a significant incentive to make the most of them. Most lads I've known have picked up a guitar at some point, learned a few tunes from tabs, and then given up. Learning an instrument like guitar involves climbing a learning curve like a rock face, and you need some serious motivation to scale it. Having a teacher can be a powerful motivating factor, and you will very likely learn more, and more quickly, than left to your own devices.
Failing that, if you've got a mate who's a decent guitarist, bribe them with some beer every once in a while and get them to show you the ropes. Again, they should notice if you're venturing down a bad path with your technique.
I've been playing guitar for almost 15 years and I've lately had the same question running through my mind. I was never particularly dedicated to practicing or learning, so my technical ability is decent but nothing special, and I have very minimal theory knowledge.
I figure the best way to get back on the path is to start out as if I'm a beginner and work up from there. I've heard good things about Justin Guitar (linked to above) and the Learn and Master Guitar series (easily 'obtainable').
As a complete beginner, a teacher is definitely going to accelerate your progress quicker than self-teaching will, but make sure you try out a few and find someone who will set you goals and teach you the fundamentals in a logical and progressive manner. A lot of teachers will just teach you songs and jam with you, which is fun, but it won't take you very far.
