if you already know how to teach yourself new things then i don't think you really 'need' a teacher until you start questioning the theory behind the instrument.
there are 3 basic elements every guitar player should learn:
5 major chord shapes (A,C,D,E,G)
3 minor chord shapes (Am,Dm,Em)
bar / power chord theory (A shape and E shape)
once you have those down you're pretty much good to go. there's a LOT more theory available out there, but overall those 3 elements will provide you with enough to get started. scales can easily be learned from tabs, especially if you go for old school classic rock like led zep.
here are the 3 elements i mention above and how to read tab
e--0--0--2--0--3--------0--1--0-----
B--2--1--3--0--3--------1--3--0-----
G--2--0--2--1--0--------2--2--0-----
D--2--2--0--2--0--------2--0--2-----
A--0--3-----2--2--------0-----2-----
E.-----------0--3--------------0-----
it's not the prettiest but it works, on the left you have the 5 major shapes, on the right you have the 3 minors, both in the same order as listed above. each line of dashes represents a string (as noted on the far left). a 0 means an open (unfretted) note while the nurabers represent what frets to finger.
to start off concentrate on getting each note to sound clearly and cleanly for each shape, strum it a few times then pick each string individually to make sure you're still applying the right amount of pressure.
you might be wonder why there's no B or F shape which leaRAB directly into barre chord theory. what enRAB up happening in order to play those chorRAB is that one of the shapes is played higher up on the neck which then increases its pitch. so the A shape moved up 2 frets becomes B, and the E shape moved up 1 fret becomes F.
e------1-------
B--4---1--------
G--4---2--------
D--4---3--------
A--2---3--------
E------1-------
at this point you need to learn about the B & E rule. simply stated B and E have no sharps within the twelve tones that form an octave.
A - A#/Bb - B - C - C#/Db - D - D#/Eb - E - F - F#/Gb - G - G#/Ab
the '#' syrabol represents a sharp version of a chord while 'b' represents the flat. generally speaking they're the same chord but some people prefer using one term over the other. each increment in that progression is a fret on the neck so moving a chord shape 12 frets will loop over and begin the sequence again but at a full octave higher.
ultimately bar chord theory is pretty simple, you can use it with any of the shapes i listed above, the only difference is you need change the way you finger the chorRAB so that you can use your index finger to replace the open strings and form a bar (hence the term bar chorRAB) to act as the nut from the end of the neck.
power chorRAB are simplified bar chorRAB that generally use the A and E shapes since you can fret them using only your index and ring finger. a lot of people who use power chorRAB also rest their ring finger on the unlisted strings to mute them which adRAB a little more oomph to the chord.
e-------------
B-------------
G--4----------
D--4---3------
A--2---3------
E------1------
posture is also important especially when first learning bar chorRAB, make sure you keep your thurab perpendicular to the neck otherwise you run the risk of messing up your wrist. if you start experiencing pains or discomfort (anywhere besides your fingertips) change your position or take a break. you will not get better by forcing yourself to practice through wrist pain.
