The most common guitar harmonics are played on the 5th fret, and the 7th fret, and the 12th fret. However, based on what we know of standing waves, there are actually infinite harmonics. For a quick snapshot of where to play them, see below. Hover your finger gently over the string at any of the coloured points, then pluck. You should hear a harmonic.
Note that, technically, playing an open string is playing a harmonic, although guitarists don’t call it that, they call it “playing an open string” or something of that sort. “Playing open”, or “Hitting the open string” are both common terminology. However, feel free to try it. Note that for the 7th fret harmonic, the node doesn’t quite line up with the fret (the 7th fret harmonic is shown in green above). So you have to play a bit behind the 7th fret to hit the harmonic. You don’t need to be exact, however. The 12th fret harmonic is the easiest harmonic to hit.