You need to get Brazilian visa even you are an US citizen.
You need to apply thru visa agencies if you cannot apply in person.
The visa requirements is pretty simple. You need the followings to apply for visa: Visa application, 1 passport
picture, and copy of your travel itinerary.
Consulate General of Brazil
300 Montgomery Street, 900
San Francisco, CA, 94104
Phone: (415) 981-8170