Which countries use dollars?

There's quite a lot. These are just a few:

United States
Canada
The Eastern Caribbean
Australia
New Zealand
Hong Kong
Ecuador
The Bahamas
Panama

It's important to remember that even though each country may call its currency a "dollar" they're not all interchangeable. For example Hong Kong dollars are worth only a small fraction of US dollars. Think of it as two different people with the same name. They both may be called Jim Smith but one might be short and thin, and the other large and heavy.