The most important things to remember when playing are to keep your hand loose, avoid unnecessary movements and finger spreading, and not to smother the strings. Having good flexibility in your hand is one thing, but trying to reach too far can be exhausting. Keep your fingers tight together, but not cramped. In general, when playing acoustic instruments you should always use the tips of your left hand fingers and not the pads to press the strings. If you use the pads, you risk muffling the sound coming from adjacent strings, which may be required to be heard. The greater sustaining properties of electric guitars often requires that such strings be damped so this rule does not always apply. Ideally your left elbow should be extended from your body, and your left hand should curl in towards your body. Your fingers should be like little hammers hitting down on the strings, and this way you will use the tips to push the strings down into the frets.
Regardless of where you are playing on the fretboard, you always have to make sure that you’re pressing down in the best spot to get the best sound. You should always be fretting down the string slightly behind the fret of the note you want to play. Press the string down firmly to the fretboard, close to the metal fret. If the finger is too far away from the fret, then the pressure is not sufficient to press the string down completely on the frets, and the note will buzz. If you are pressing too close to the fret you will sometimes accidentally play a note too high. You’ll have to practice to get the right amount of pressure to use and the right distance at which to hold your arm.
Content Credit: https://en.wikibooks.org/wiki/Guitar/The_Basics