All notes must be fretted with the tips of the fingers and positioned as close as practical to the fretwires.
There are two basic positions for the left hand. In most cases the thumb should be positioned behind the neck of the guitar with the fingers evenly arched over the fretboard. When using techniques such as the bend, release bend, vibrato etc. you may find it more comfortable to have the thumb in a higher position, wrapped over the top of the fretboard.
The notes on the guitar fretboard change alphabetically one fret at a time. You will recall that the musical alphabet is made up of seven letters: A B C D E F G.
The distance between most of the letters is called a tone (or whole step). From A to B is a tone, from D to E is a tone etc. There are even smaller steps between these letters called half steps (also called semitones). They are written as follows: A A# B C C# D D# E F F# G G#. These are the 12 notes used in all western music.
Notice that there are no steps between B-C and E-F; the distance between these notes is already a semitone. The signs following the non-bold letters are called sharps. Flat signs can also be used, giving: A Ab G Gb F E Eb D Db C B Bb.