Driving in Brazil is likely going to be challenging for most, and should be avoided. Language barriers, theft, robberies, and accidents are all reasons to avoid driving.
Driving isn't as hectic as some south american countries, but it's still challenging. If you have never been to Brazil and are still thinking of driving, I would recommend arriving without any reservations, giving yourself a day to adjust and then decide if you need a car. If you do, go online and book one and then go pick it up. Have a taxi take you to your hotel or housing accomodations the first day.
You must have an INTER-AMERICAN DRIVING PERMIT, which is different from the international drivers permit! Same forms, but you get a slightly different document. Don't even try and drive in Brazil without a proper license! In the US or Canada, you can go to AAA. You will need to fill in a simple form, and give them two passport style pictures. The permit costs around $15 and is done on the spot. If you rent a car, you can't take it out of Brazil to visit countries around south america either!
If after the first day you believe you want a car, here are some sites we have used to rent cars: