When I first started playing as a young teen my parents didn't want to get me lessons so I tought myself for about 4 years, in my opinion the time was very valuable to my growth as a player.
Because I wanted to learn I sought after knowledge wherever I could find it, friends, books the internet etc. it also made me value the information I got greatly and I became obsessed with exploring and understanding the guitar on my own. I perhaps developed some bad habits but they were easily corrected when I got a teacher at college later on, the ability to teach my self never left and became highly developed. A teacher is only with you one or two hours a week if your lucky, you are with yourself 24/7, not knowing how to properly teach ones self is a major detriment to skill developement in any discipline. It also allows you to become more critical of your teachers, they have problems and idiosyncrasies of their own, and knowing how to spot those is helpful to getting the most out of them.
That being said, the most important advice for teaching yourself in my opinion is:
Get an electric tuner and always play in tune, learn how to tune on your own as well but the electric tuner is much easier for the beginner.
learn chords and scales, play some rock and pop music, it allows you understand positions better. Classical guitar is not the natural state of guitar playing, it's an accompaniment instrument primarily, it's easier to develope the basic skills of music with simpler music. Especially rhythm, it's the primary component of music, even classical.
Be critical of yourself, examine everything about your playing, how your fingers feel, the level of pressure and most importantly the tone and the sound.
Try to develope the ability to hear in your head what you want to hear out of the guitar, the mind is what is really playing the instrument, the fingers and guitar are just extensions of the former.
Be obsessed, perhaps the most difficult aspect is that self teaching requires more work, when I started I was playing or learning every chance I could, if I was unable to play I was thinking about it, playing in my head, tapping rhythms etc.
Listen to a ton of music, only compare yourself to professionals. How can you know how you want to play if you don't have any reference?
Play with other musicians, like I said guitar is for accompanying primarily, your rhythm and musicality will grow tremendously when you play with others.
Record yourself, in the year 2017 there's no excuse for not being able to record yourself and listen back, it is vital to understand how you actually sound when your not distracted by playing the guitar.
And most importantly have fun and love playing, if you don't how will you ever improve?
I suppose that's my 2 cents on the topic, my final note is to understand what you want out of your playing, if your doing it for your own enjoyment and playing a bit for others self teaching is fine. If you want to be a professional, you will most likely eventually need a teacher.