Until they build the new one, there's only the one in Cambridge (the others are way out in the suburbs), in the Cambridgeside Galleria. I've spent some time there, it's decently nice, though far from a big one.
If you're actually in Boston, the easiest way to get there is to take the Green Line to Lechemere (doesn't matter which line in that direction). The mall (galleria...) is only about two blocks from the station.
If you're out on the Red Line (Cambridge, Somerville, Arlington) it would be much faster to take the Red Line to Kendall Square (MIT) and walk. It's a longer walk, but a pleasant one.