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.