Hi stella,
The best beach points in Brazil are Ilha Grande, Florianopolis and Busios. Rio de Janeiro also has good beaches combined with a busy/cool city. Salvador Bahia also has great beaches in the city and the surroundings (this is where capoeira and samba where born). Basically, all the coast of Brazil are filled with beautiful paradise-like beaches.
But, I have been to Brazil 2 times now looking for a beach vacation, but it ended up raining almost half of the days. Brazil is a tropical country so it basically rains 50% of the time no matter what month/season you go. If you want a guaranteed beach vacation I would actually consider going to the Caribean during no-rain season. But Brazil is not only about the beaches, its culture and lifestyle are contagiuos. If you go with the right mindset, you wont mind if it rains.
hope this helps
Richard Leon
Travel Advisor
www.peruforless.com