There is no such thing as "avoid tones". Just a bad melody or a bad voicing (harmony). Like the example you said you shouldn't play F over a Cmaj7 chord. This is probably one of the most common notes that would be played, even on a strong beat. It's called a suspension, they normally resolve down. So F over a Cmaj7 chord would resolve to E. Or G, which is called a retardation. One thing though is you shouldn't over use suspensions, too many can make the melody sound weak. But when used properly they add strenght to a melody.

About the dissonant sound, with the 3rd. Usually when the 3rd is played in one voice, while the 4th is in another voice range. I'll explain better, say you play the C maj7 chord, the E is found on the 2nd fret of the D string. F will probably be played on the 1st fret of the high E string, or anywhere else on the fretboard (same pitch). There is dissonance, but in music dissonance plays the equal counter-role of consonance. Music that's all consonant is like watching a movie with only one actor, BORING!!!