Yeah, you'll need a CharlieCard. You can get one from one of these locations in Waltham, you don't need to go to the "CharlieCard Store" downtown or anything - that 7-Eleven will do. Load up like $20, and then 'tap to pay' when going on a bus or subway. It's that easy.

They're soon piloting ApplePay or Credit Card Tap & Pay for the T, so hopefully that'll be standard next year or the year after.