Learn a D major. This chord only requires the bottom four strings. Place your index finger on the 3rd string, 2nd fret. Your ring finger then goes on the 2nd string, 3rd fret, and your middle finger is the 1st string, second fret. You'll form a little triangle shape. Only strum these three strings and the 4th string -- the open D -- to sound out the chord.
Unlike many string instruments which use a bow to create a sound, guitars are played by plucking or strumming the strings on the instrument, which vibrate to create a sound. Chords are played when the guitarist holds their fingers down on more than one individual string to change its length, which in turn changes the frequency of the vibration created - in other words, it creates a different note. Performers often use guitar tabs, rather than sheet music, when learning music: this acts as a kind of 'map' to teach guitarists where to put their fingers on the strings.