Easy. Take Rt2 East to the Alewife T-station exit. It will cost you only $4 to park, and $2 for tokens there and back. Take the T to the Park Street station, and from there hop on the Green Line train to Lechmere (almost every train goes there). Get out, walk a few blocks, and you are there, economically, safely, and with no traffic jams to fight!
Enjoy the game! Go SOX!