You will need to go to a City or Town clerk's office in Massachusetts to file your intentions. Both of you need to appear in person with very few exceptions (military, illness or incarceration). The fee for filing your intention in Westford is $25.
There is a 3-day waiting period before you may obtain your marriage license (the waiting period may be waived by court order), so don't wait until the very last minute for this. The marriage license is valid for 60 days from the date of filing, so don't come in too soon. You will be swearing or affirming that there is an absence of any legal impediment to marriage in Massachusetts or your resident jurisdiction after marriage if not Massachusetts. For additional information view the Legal Impediments (PDF).
Non-resident out of state clergy members must petition to solemnize a marriage in Massachusetts, view additional information from the Massachusetts Secretary of the Commonwealth. If the officiant performing the ceremony does not have a religious affiliation and is not a Justice of the Peace, a 1-day marriage designation is required.