To fret a note on the guitar means to press the tip of your finger down on a string, while keeping your knuckles bent. Try to get the fingertip to come down vertically on the string rather than from the side. This position exerts the greatest pressure on the string and also prevents the sides of the finger from touching adjacent strings, which may cause buzzing or muting (deadening the string, or preventing it from ringing). Use your thumb from its position underneath the neck to help "squeeze" the fingerboard for a tighter grip.

Building up left-hand strength and facility takes time, and you just have to be patient. Don't try to speed up the process through artificial means. Hand-strengthening devices designed for expediting left-hand endurance are dubious, and at best, marginally effective. The same goes for the homegrown method of squeezing one of those spongy "therapy" balls — okay for strength, but only in that it makes you better at squeezing stuff rather than fretting notes and gripping chords. Nothing helps build your left-hand fretting ability better and faster than simply playing the guitar.