Unless you are traveling for a specific timeframe around the Olympics, you need a visa if you have a US passport.
The Brazilian Consulate in Los Angeles has jurisdiction over residents of San Diego.
This is one page of their website regarding visas. Read it carefully and thoroughly. There is more information about the visa process on other pages.
http://losangeles.itamaraty.gov.br/e...urist_visa.xml
You need to fill out an online form, provide documents and a photo, and pay U$160. The website will tell you how and when to submit the documents and pick up your visa.

The cheapest time to go is when the dollar is doing well against the real.
Air fares are a bit higher from the beginning of December until after Carnaval.