When the Red Sox are playing at Fenway Park, getting into the area is extremely difficult. While the MBTA does run extra service, expect Green Line cars to be filled to capacity, and be prepared to wait for several trains before you're able to board. Parking garages fill up quickly, and rates are exorbitant: during playoff games, some parking lots have been known to charge up to $100 for parking.
By Subway
The B, C, and D branches of the Green Line stop at Kenmore, and the D branch continues on to Fenway. Despite the name, the closest station to Fenway Park is Kenmore, not Fenway. The E branch stops at Symphony, Northeastern, Museum of Fine Arts, and Longwood Medical Area stations. The Longwood Medical Area station is separate from the Longwood station on the D branch of the Green Line: the two stations are neither related nor interconnected.
The Orange Line stops at Ruggles station. This station is several long blocks away from Fenway Park. If you're going to a Red Sox game and want to avoid the crowds on the Green Line, however, it may be worthwhile to take the T to Ruggles and transfer to the 8 or 19 bus routes to get to the ballpark.
By car
From Storrow Drive, take the Kenmore Square/Fenway exit, and follow the signs for your destination. Those going to Fenway Park should take the Kenmore Square exit. There are a number of parking garages in the area, particularly around Fenway Park.
By bus
MBTA bus routes 8, 19, 55, 57, 60, and 65 all stop in the area.