You may take out cash at ATMs such as Citibank or Banco 24 horas. Always have some cash on you, not too much yet nor too little. The rule is that you should never go out without any cash on you. If you get mocked you want to have something to give away and spare you of any further pain.
Most public places in Brazil, such as museums or office buildings require an ID card to enter. Therefore always walk with your ID card (not your passport). If Id cards are not common in your country, take a copy of your passport, but be aware that it might not be accepted everywhere. In that case, you may actually need your passport, but don’t bring it to the beach or a party.
when going to a party or the beach a copy will do. Need to get a copy? go to a Paper store (Papelaria) and ask for a "xerox" of your passport.