Which of these is the first letter of the Greek alphabet?

Alpha is the first letter of the Greek alphabet. In the system of Greek numerals, it has a value of one. Alpha is derived from the Phoenician letter aleph aleph, which is the West Semitic word for "ox". Letters that arose from alpha include the Latin letter A and the Cyrillic letter А. In Ancient Greek, alpha was pronounced [a] and could be either phonemically long ([aː]) or short ([a]).

Source: Wikipedia