The caucus for Barack Obama is at Jordan Hall on Saturday April 5, and the doors are locked at 1:00pm, so be there before then.
There's more info here about the process, as well as the locations of the caucuses for those of you who aren't in the 8th congressional district. FYI, the 8th district includes Cambridge, Somerville, and most of Boston except for South Boston and parts of Dorchester, West Roxbury, Roslindale, and Hyde Park. To find out what district you're in, go here, and scroll down to District Representatives: Congressional. If yours is Michael Capuano, then come to the caucus with me, dammit.