Alphabet (a set of letters) is first found in hieroglyphs, in Egypt, around 2700 BC
http://en.wikipedia.org/wiki/Egyptian_uniliteral_signs
Howerver, the oldest scripts found are the Sumerian cuneiform scripts, ~3000 BC, but there's no real Alphabet used there (signs are representing words and not letters)
http://en.wikipedia.org/wiki/Cuneiform_script
