So you are running away from the cold for a while?
Both Steve and Gervais does give you very good advices, to have a car or not also depend on what you are going to do in Tobago, and how long you are going to be here.
The bus is more trusted now a days, but there´s no specific time table to read, locals can tell you when it is supposed to come, but there no "just-in-time" here in Tobago! And you must buy bus-tickets in Black Rock, not ON the bus, they don´t sell tickets on the buses here.
Gervais did tell you the bus route for that area, but I must point out one important thing. Let say you would like to go by bus to an area, outside what Gervais say, then you must go to Scarborough first. Almost all bus routes start and end in Scarborough, so to travel by bus from the Black Rock area down to (example) Crown Point, you maybe first have to go to Scarborough and than take a new bus down, that can mean waiting for buses! I´e seen buses in Crown Point with a sign "Black Rock" but I don´t know any time table, myself mostly walking on the road.
Depending on how long you are going to stay in Tobago, and what you want to see and do, a rented cars might be the best. It might be that you start your vacation in Black Rock, walking around, spend time around there and on the very big beach, get some tan (it´s a strong tropic sun here) and when that is done you want to see more of Tobago, because there much more to see and discover.
Black Rock Beach is big, not many use that beach, there´s a beach restaurant there, own by the hotel so do not expect cheap prices. Better to buy what you need for the day at the shop in Black Rock.
As Gervais mention Grange Bay, a word of warning. It´s not a beach with life guards, and there´s sometimes very strong currents there, one Trini did die there last week, and he was a good swimmer!
Don´t take a chance, use Mt irvine beach instead than. There are two such beaches, to local beach which has a local restaurant/bar and showers/toilets, but it has a lot of smaller stones in the sea. The other one is the hotel beach, but everyone can use that beach. The sand is much better there, and the hotel has a bar/restaurant there.
There´s much more to tell, but I don´t want to write a boook here! Feel free to write to my own e-mail address if you want that: email@example.com
Hope you will like your stay and that you will be back some other year.
I have been travelling to West Indies since late 80´s, to Trinidad since 1990, Tobago since 1992.