Here it is written in diamond notes in the first measure. How should one play them on guitar and also what do they indicate?
The diamond notes indicate harmonics. Basically you generate a note by lightly touching the string an integer fraction of the way down (like 1/3, 1/4, 1/5 etc.) so that instead of vibrating with the frequency length of the string it vibrates with the fractional length.
In this specific example this is natural harmonic played by placing finger above 7th fret (see text above the score) of three bass strings.
There is some more useful information in a good answer to this related question on this forum.
The Wikipedia article referenced above gives some additional information on ways to play harmonics on the guitar which don't apply to other stringed instruments:
Tapped harmonics were popularized by Eddie van Halen. This technique is an extension of the tapping technique. The note is fretted as usual, but instead of striking the string the excitation energy required to sound the note is achieved by tapping at a harmonic nodal point. The tapping finger bounces lightly on and off the fret. The open string technique can be extended to artificial harmonics. For instance, for an octave harmonic (12-fret nodal point) press at the third fret, and tap the fifteenth fret, as 12 + 3 = 15.
A pinch harmonic (also known as squelch picking, pick harmonic or squealy) is a guitar technique to achieve artificial harmonics in which the player's thumb or index finger on the picking hand slightly catches the string after it is picked, canceling (silencing) the fundamental frequency of the string, and letting one of the overtones dominate. This results in a high-pitched sound which is particularly discernible on an electrically amplified guitar as a "squeal".
Adding to Brian's answer: the instruction on top at that point is 'ar. 7'. Meaning play the 'armonic at the 7th fret. (Could well have been coined by a Brummie!).
The diamond headed notes are the original notes, not the ones which will sound. Here, they are the bottom 3 strings, open (D-tuned bottom string). Making an A5 'chord', due to that being the 2nd partial/3rd harmonic.
Simple way to play - place a barre finger just touching the 3 strings over the 7th fret, pluck with 3 fingers from other hand. No need to move them off quickly after they sound - like a lot of players believe.