By car
From points North: You can get to Somerville in two ways:
take Interstate 93 and get off at exits 29-31
take Route 3 and park at the Alewife station on the MBTA Red Line. Take the inbound Red Line to Davis Square. This is advisable on the weekends, as parking can be hard to find near Davis Square.
By train
Take the MBTA (http://www.mbta.com) Red Line to either the Davis Square or Porter Square Stations. If you're coming from downtown e.g. South Station you'll want to hop on an Alewife/Harvard bound train.
You can also take the Orange Line to Sullivan Square and then take one of the MBTA busses up Broadway through Winter Hill and beyond.
Should you be coming from northwest of Boston e.g. Fitchburg you can also ride the MBTA Commuter Rail.
By plane
Fly in to Logan International Airport in Boston. A taxi from Logan to Somerville can cost anywhere between $20 and $35 depending on the route taken and time of day. You can also take the MBTA Blue Line subway from Logan to either the Orange Line switch at State Street or the Red Line. Alternately, you can take the Silver Line bus service to South Station, then transfer to the Red Line. The subway ride takes longer than a taxi and may involve some walking, which is something to consider when you have lots of luggage.
By bus
The MBTA bus system will take you from Sullivan Square Orange Line through Somerville on to Arlington center and Medford. You can also get to Somerville by bus from Lechmere Station at the end of the Green Line in Cambridge, near the Cambridgeside Galleria and Boston Museum of Science.