Basic music theory, as far as I'm concerned, is a necessity for any guitarist. Whenever I play with a guitarist, and I say "This is in the key of Em" they had better know that that means: E-F#-G-A-B-C-D-E. If not, I'm out of there. Anyhow, I agree with Azreal, saying that a teacher doesn't help or is in fact worse for the beginning stages of guitar is a big pile of bull, Digit.