I can give you some basic information, as long as you understand that all personal injury cases are different, and the procedure will vary slightly from case to case.  In general, after you select an attorney, he or she will sit down with you and talk about your injury, collect information, and prepare the case and the "Complaint" for filing at court.  If insurance companies are involved, your attorney may negotiate with them prior to filing the Complaint at court.  Unless your attorney reaches an agreement with the Defendant or the Defendant's insurance company, he will then file the Complaint at the appropriate court. Your attorney will also arrange to have the Complaint and Summons served on the Defendant.  Follow this link to a post with my thoughts on finding Massachusetts personal injury lawyers. 

At this point, the Defendant must either file an Answer to the Complaint or a motion, such as a motion to dismiss, which may have the effect of delaying the deadline for the filing of the Answer to the Complaint while the court considers the motion.  Negotiations usually continue during this time, and the parties may begin discovery. 

Discovery, such as requests for answers to interrogatories and depositions, is the process through which both the Defendant and the Plaintiff may gather information about the injury claim, about the available evidence, and about the people who may testify and what they will testify to.  Depositions, in which a witness or party to the suit is questioned by an attorney in front of court reporter, is a very common form of discovery.  In cases involving personal injury, it is also common for the Defendant's attorney to seek a medical evaluation of the Plaintiff.  Depending on how complex the case is, and on how busy the court's calendar is, the discovery phase may last for one or more years, until the parties reach a settlement or the case goes to trial.

If you reach a settlement with the Defendant, you will sign a settlement agreement with a release and, presumably, receive payment for your damages.  If no settlement is reached and the case goes to trial, then your attorney will have to present evidence to demonstrate that you were injured, that the Defendant caused the injury and resulting damages, and that you are entitled to some specified amount in damages.  The Defendant may challenge or rebut your evidence and may also present his own evidence.  The finder of fact (judge or jury) will then decide the matter.  Keep in mind that a good personal injury attorney will explain all this to you, in even greater detail, and will keep you apprised of all developments in the case.  Good luck.