Many beginning guitarists have trouble with achieving a correct guitar left hand position. Later on, this can be a significant roadblock in a guitarist's progress. Working to get a correct guitar left hand position now can make life much easier when you get a little more advanced.
Most guitarists switch back and forth between two guitar left hand positions. Which one you choose depends on what you're playing, the shape of your neck, whether your standing or sitting, and other factors. You can choose between the "thumb-over" or the "classical" position. The thumb-over left hand position is useful for blues and rock lead guitar which relies heavily on note-bending. This guitar left hand position can also make some chords easier to achieve. This is the position that feels most natural for many beginners. To use it, place your hand on the neck with the thumb sticking up above the top side. The lower knuckles of your fingers, on the top part of your palm, can rest on the neck. This lets you use a great deal of muscle for bending notes, minimizes the number of frets you have accessible because the fingers can't stretch as far.
For the classical guitar left hand position, the tip of the thumb should be at about the middle of the back of the neck. The palm should not be in contact with the neck and the knuckles should be parrallel to the bottom edge. The fingers should be roughly parallel to the frets and should be arched over the fingerboard; not flat. This guitar left hand position may feel awkward to many beginners, but it is preferred for fast technical passages and anything requiring a wide reach. This is especially useful for classical and heavy metal "shredding" styles. Failure to master this guitar left hand position early could greatly impede your technical progress down the road.
Regardless of which guitar left hand position you choose for a passage, their should never be unnecessary tension in the hand. One common mistake is to use too much pressure on the strings to fret a note. It doesn't take a great deal of pressure for a fret to stop a string. It's also good to avoid contorting the thumb into unnatural positions when playing barre chords or chords with a wide stretch. These sources of extra tension may not seem like a big deal for now, but they can cause problems in the long run.
You should start practicing correct guitar left hand positions as soon as possible. In the beginning you may find yourself slipping back into more comfortable positions, but if you catch them now it will pay off further down the line.