Before you go to the local consulate to obtain your visa, make sure you have a faxed itinerary from the hotel(s) you will be staying at during your visit. (Call them and set up a reservation, and have them fax you the details on letterhead.) If you are going to be staying with a family here, bring the letter with the invitation. It must state they know you are coming, not just "come visit anytime".
Also bring your round trip ticket. (Make sure you buy a refundable ticket, just in case you are not approved. It is a little more expensive but it will be better than losing everything if there is a problem.) If you do not have the plane ticket when you go into the consulate with your passport and itinerary you will not get a visa.
I do not know what sort of paper you can get from the police where you live, but get anything you can stating that you have no criminal history.
If you show up with these documents, you will almost always be approved.
The US consulate here is hard on Brazilians, even wealthy tourists just going to Disney or Miami, so the Brazilian consulate will not make this easy if you do not follow the rules.
Learn a few words of Português, even if you are just going to Rio.
Seja bem vindo