It is the Complaint that officially begins a personal injury lawsuit in Massachusetts. This document is referred to as a "pleading." If you have a small case with limited damages (personal injuries) the case should be filed in the District Court. If you have significant damages, then the Superior Court is the proper court to commence the lawsuit.
After you file the Complaint, as well as the required Statement of Damages in court, you must then have the Complaint "served" on the defendant(s). The best way to "effect service" is to have a sheriff serve the Complaint on the defendant(s). This will cost, depending on the travel time for the sheriff, around $50.00.
You must "serve" the defendant(s) within ninety days of the filing of the Complaint in court. Once service has been made, the defendant(s) has twenty days to file an "Answer" in the court in which the Complaint was filed. You must send to the court the "proof of service" which shows that you properly served the defendant(s) with the Complaint. It is then that "discovery" begins.
If you have a Massachusetts personal injury case, I STRONGLY suggest you contact a local Massachusetts personal injury lawyer. The litigation of Massachusetts personal injury claims is laden with traps for non-lawyers who simply do not know how to navigate the litigation process.
Best of luck with your Massachusetts personal injury claim!