The easiest way to play a note once you've got your hands on a guitar is to pluck one of the strings while it's open, being careful that nothing is touching it anywhere along its length because that would change the sound it makes.
Unfortunately you can only play 6 notes in this way (because there are 6 strings on the guitar) and those notes don't even go together very well. To play any more notes you need to use the frets to change the sound the string makes.
The frets are the metal strips that go across the neck of the guitar. By putting your finger on a string just behind one of the frets you can change the note the string makes. You have to press quite hard but the closer your finger is to the fret in front of it the easier it is to get a clear note.
You can put your finger behind any of the frets on any of the strings, but this diagram shows the 2nd fret of the 5th string being played, and where to put your finger to get the clearest sound.
It should ring for a few moments and sound similar to when the string is being played open. Don't worry if it doesn't sound like that at first, it'll keep getting easier for as long as you play guitar but it should only take a few minutes before you can get it sounding ok without too much of a struggle.
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