"Paper or plastic?"
Charlie Tickets are paper surfaced with a magnetic stripe. The ticket machines issue these either in stored value (so many dollars worth) or multi-day (so many consecutive days) versions. Insert the card into the slot at the turnstile or on a bus fare box.
Charlie Cards are plastic and do not have a magnetic stripe. (There's a little thin computer chip inside.) You need to go to specific locations, namely some banks and convenience stores (I don't remember which) to get one. Sometimes a station agent will be doing a promotion and handing them out. They are for stored value (put on so many dollars worth of fares). Press the card on the black circle or rectangle about the same size on the fare box or turnstile, don't put it in a slot. Charlie Cards offer some per-ride fare discounts and bus transfers that Charlie Tickets do not offer.
To buy a ticket to ride the subway including light rail, do not press the "commuter rail" button on the machine. Choose subway/bus.
You can add more to stored value Charlie Tickets and to Charlie Cards whenever you wish using the same machines.
Even if the value is used up, hold on to the Charlie Ticket until the end of your ride. You may have to show it to a conductor or a station agent.
Wait for people near the turnstiles and exiting the station before going in.
If you haven't paid yet, enter only the front door of a light rail car (trolley) when using a stored value card; enter any open door when using a multi-day card.
On the Green Line, if your trip takes you through Park Street, take any train there and change to the actual train of your choice there. (If you wish to, you can wait at your original station for the train of your choice if one is supposed to stop there.) Even experienced T travelers get crossed up when a sudden or ad hoc schedule change is made without notice and trains that normally serve a particular station get turned back sooner.
Once in awhile, a portion of a subway line is out of service due to a mishap or for maintenance and passengers must transfer substitute buses. It is customary not to get out your fare card when transferring to these buses and, if you are to transfer back to the subway, you walk through an open turnstile gate without getting out your fare card again.
Disney hints:
http://www.cockam.com/disney.htm