Watch the Fingertip Pressure online guitar lesson by Bruce Arnold from Guitar Physiology Survival Guide
Don't press too hard with your fingers. This is a common problem especially for beginners and for students playing an acoustic guitar. Most students don't even realize how hard they are pushing. Try pushing your finger down on a string, slowly on a count of ten while picking it, so that by ten, the note first sounds. That will give you a good idea of just how little pressure is really needed.
Playing a whole scale like this will really help you to realize how little pressure you need to make a note sound.