First, you need to determine what type of guitar you have - acoustic nylon, acoustic steel string, or electric. You want to be sure to use the correct strings for your particular guitar. Acoustic guitars that require nylon strings, such as classical, flamenco and some folk guitars, generally have lighter tops, or soundboards, with less internal bracing than those found on steel-string acoustics, and stand the risk of serious damage if fitted with steel strings. Steel-string acoustics are designed to withstand the added stress that steel strings exert on the top, bridge, nut and neck, and won't sound very good with nylon strings, if they even fit. Electric guitar strings must be made of ferromagnetic metals like steel and nickel, so they can interact with the magnetic pickups, while acoustic-electric guitars typically use a different type of pickup which senses vibrations from the bridge, so acoustic strings may just have a steel core wound with a phosphor bronze alloy wrap for bright tone. Guitars with whammy bars might require a few extra steps to keep everything stable, so check your manufacturer's instructions or look for online videos.
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.
Guitars have been played since the Renaissance era, after descending from the ancient Greek instrument, the kithara. Or, at least, that's how the theory goes: the only real proof for this is the similarities between the Greek word and the Spanish word, quitarra. Early guitars often had four strings, evolving into the six-string version we know today in the late 1600s. Anton Stradivari, the famous violin-maker, also had a hand in making guitars. There's now only one Stradivarius guitar left in existence.