It's essential to learn guitar finger positions for notes and chords. This is basic to successful playing, so spend some time getting used to this.
This diagram shows how and where to place your finger on the string, to get a clear sound.
I am playing the note "E".
Notice how my second finger is almost perpendicular to the fretboard.
It does not touch any other strings, and it's placed slightly behind the second fret.
Only my fingertip is pressing the string.
There are several fingering mistakes in this diagram.
My finger is on the wrong angle, causing me to touch the neighboring strings. Touching the other strings stops them from playing.
Also, my finger is touching the fret, which causes the note to thud and buzz.
Play an "E" on the D string at (behind) the second fret. Keep your finger as close to the fret as you can without touching it. Be sure to only use your fingertip.
It's easier to accomplish this with your thumb almost centered on the back of the guitar's neck, not wrapped around it.
Pluck the string using only the very tip of your pick, and listen to the tone.
It should be clear, without buzzing, and should ring for a long time.
If you don't use enough finger pressure the note won't ring long. Too much pressure, and your fingers will get sore and you'll play slower.
Practice getting just the right pressure by pressing firmly on the string until you get a clear sustaining note, then, slowly release some pressure until the note starts to die out. Do this several times until you get a feel for it. Use the least amount of pressure you can, while getting the note to ring clear and long. I can't stress enough how important this is, always try to push down as lightly as possible.
Next slowly play the "E", and the open strings on either side. All three strings should ring out clearly. If they don't you're touching a string that you shouldn't, because you aren't using the correct guitar finger positions yet. Adjust your finger position until all three notes play well.