To get a waiver on the sales tax, you need to fill out a sales tax exemption form. You also need to have proof of car insurance. Your insurance agent needs to complete, stamp, and sign an Application for Registration and Title (RMV-1).
Take both forms, along with your signed title (be sure the date sold, odometer reading, and the purchase price have been filled out on the back), to a full-service RMV center. The new owner will be issued a registration and plates and will be mailed a new title.
Source: http://www.mass.gov/rmv/regs/index.htm