Depending on the country of your passport, a visa might be required for you to enter Brazil. In this matter, Brazil adopts a policy of reciprocity, requiring a visa for citizens whose countries require the same from Brazilian citizens.
The following link will lead you to a bilingual webpage of the Brazilian Ministry for Foreign Affairs (Itamaraty):
It is very likely that you will find there the essential information regarding whether or not you must hold a visa to enter Brazil. From that same website we have extracted the following official list for visa requirements. For citizens of all countries marked with number 1, a visa is mandatory.
If you need an invitation letter to apply for a visa, please send an e-mail to vortex (at) iip.ufrn.br