The Government Center garage will get you on and off 93 fairly quickly, and is a 5-10 minute walk to the Garden. But they recently upped the price from $20 to $25, which makes it a little excessive unless you've got a carful of people to split it with (which you don't). Also a bit of a line getting out, depending on the crowd that night. It's what I use, but I'm not totally familiar with the area either. If you are hanging out in the area after the game for a bit, the line will die down, but there are probably some cheaper/easier options out there.