Eeka figured she'll use the MBTA trip planner to find the best way to get from Forest Hills station to Beech Street in Roslindale. She discovered a slight problem - the planner thought she wants to get to Beech Street from Forest Hills in Queens. And apparently that line isn't finished yet (what would they call it? The Orange G? The 34 F?).
In the meantime, as I'm sure eeka found out (or already knew), the best way to get from the real Forest Hills to Beech Street is to hop on any of the roughly 3,000 bus routes that go down Washington Street toward Dedham (and even Walpole) and then get off here (just look for the Pleasant Cafe on the right, then yell "Bus driver, stop this bus!").
Ed. skeptical note: I just tried recreating eeka's proposed itinerary, i.e., "forest hills" to start and "washington st. and beech st., roslindale" as my destination. MBTA.com replied "there are no routes within 1 mile" of Forest Hills and suggested I first get myself to Providence (148.99 miles away) or Union Station in Worcester (151.95 miles away). Guess it's a good thing I can walk to Washington and Beech from my house.