hey guys ! i need your help :D i ll be staying in sao paulo for 3 days beginning the 31st of july so would you tell me what to do ( beaches ,restaurant ; bars , nightlife , museums , parcs ,shopping maybe ) what really not to miss ! and i really need names of good and safe hostels ! thnx

Sampa is home to tonnes of Museums. I would go to the Football museum at Estadio do Paceambu which is worthwhile - when I went on a wednesday it was free. Others are MASP on Av Paulista which is free on tuesday or wednesday (ask your hostel for the days its free). There are some great ones in Parque Ibirapuera as well.
Beaches - don't bother with 3 days. Its a bit of a hike as well.
Shopping - something Sao Paulo does pretty darn well. Have a wander down Rua Oscar Freire for a glimpse of the glamourous side. Plus it has the Havaiana store there. Plenty of Shopping Centres about. They are everywhere. Again, ask your hostel for the best ones.
Try a Churrascaria at least once. Plenty of great little bars and cafes in Vila Madalena.
Speaking of Vila Madalena, stay at a hostel there. Some great ones about. Use the usual hostelbookers/hostelworld sites.