You have to make sure you have the fretting finger down behind the fret but as closely to the fret as possible. When your finger is too far behind, you're likely to hear a buzzing sound because the string isnt making constant contact with the fret. There is plenty of videos around on the web teaching you how to do this correctly.

This really shouldnt be that hard. I'm thinking maybe the problem might also be that the 'action' (distance between string and fretboard) is either too high or too low for it to be comfortable for you specifically. This can be altered by an experience technician.