To obtain an Employer Identification Number (EIN), businesses must apply online through the IRS website by providing necessary business details such as name, address, and structure. Once the EIN is received, businesses can proceed to register for sales tax in Massachusetts via MassTaxConnect. They will complete the online registration form, providing their EIN and other required business information. Upon approval, the business receives a Sales and Use Tax Registration Certificate, authorizing them to collect and remit sales tax in Massachusetts. Ensuring all business information is accurate during registration is crucial for successful compliance.