When you first start out on guitar, you will need to know how to place your fretting fingers on the fretboard. Use your fingertips to press down on the strings. Avoid using the flat part of your fingers, as it can mute other strings. Place your fingers just behind the frets. Use firm, but not excessive, pressure to get a clear sound.
Try this, place your index finger between the 2nd and 3rd fret of the 2nd string, and pluck the string. This is a C note. Find the sweet spot between the 2nd and 3rd frets. If you are back too far, you will have to press harder. If you are too close to the fret, you may get some buzzing or muting of the note.
Start slowly, placing your finger deliberately and adjusting for comfort. Try moving your finger up and down the neck. Try it on different strings. Notice any spots that are more difficult or easier to play than others? Play each note carefully and slowly to ensure a clear sound on each note.
Pay close attention to what your body is telling you. If anything feels painful or uncomfortable, you may need to recheck your hand or finger positioning.