Your question's answer is written in Go rules : Since the black player starts the game, he has a significant advantage. In order to balance it, a value called komi is subtracted from the black player score. It is 5.5 for a 19x19 board, 4.5 for a 13x13 board and 3.5 for a 9x9 board.
(hide) If you want to find an opponent with similar playing skills to your own look in the Ratings page of the type of game you want and find a player with a similar BKR. (pauloaguia) (show all tips)