I started learning guitar and I learned the correct finger positioning on the fretboard and my guitar is tuned. I don't know what I'm doing wrong.
There is not enough info to really help but here are a few possibilities.
If you are trying to play a chord then some of the flesh of one finger might be touching another string that needs to ring. It takes time to get the correct posture in your muscle memory so this will happen for a while.
If it mutes when you are playing open strings then your guitar is out of adjustment and some of the strings are being damped by hitting frets. This can happen even with chords.
Try pressing the string to the fret with less force.