forefinger-1, middle-2, ring finger-3, pinkie or small finger-4 and the thumb-T.
The letter "O" means that that string is also being played but not pressed on. The letter "X" ( in some chord charts) or the absence of any number or letter on top of every vertical line (guitar strings) means that, that string is not to be played. When you see a letter "T" on a chord pattern, it means your thumb must press that string. For now you can use a single downward strum with your thumb just to hear the sound of a chord. Be sure you place your fingers inside the fret board space and not on the fret (horizontal line) itself to produce a better sound. Take time to execute proper finger positioning, check that all the strings you pressed on and the ones that are not pressed are both sounding clearly and practice these as often as you want for your left hand to familiarize the chords.
Below is the G chord illustration. Here you can see the strings with the sign "O"(as in open strings) just above the vertical lines (guitar strings) signifies the strings are to be played.
Here we can see the symbol "X" in the C chord.