Can I work with a tourist visa? No, not legally. Will anyone ever give you a problem if you do work while on a tourist visa? No, I doubt it.
Can I change from a tourist visa to a work visa? I believe you can. You will need to have a sponsor first, of course.
Work Visa
It is very expensive and complicated for a company in Brazil to sponsor a foreigner. I do not know of any way for a person wishing to teach English in Brazil to be sponsored while they are in a different country.
If you are a professional in another area such as information technology, engineering, etc. you may have a chance of finding a company to sponsor you but then you would not be teaching English.
Should you wish to stay in Brazil for an extended period of time you might find an English school or course which would be willing to sponsor you if you pay for the entire process. This is a possibility and one that you will have to discuss with someone you trust once you have been in Brazil for a while.