FIFA World Cup visit! (Rio de Janeiro)

Hello! I live in Bangkok but will be visiting Rio in July for the World Cup!! I've never been to South America and don't know anything about it. I'd appreciate if you could provide some local insight into getting around and places to go. Mainly I'd like to know:

1. The best way to get from airport to town, and within town. We have a big group of 8 people. Do we need to book car with driver in advance?

2. Will I be able to use foreign credit cards easily? Or better to carry cash? In which currency?

3. I heard the security may not be too good there. Any advice on safety tips?

Sorry for asking such questions but I'm really clueless when it comes to South America. Please help! :-) We'll go to Chile and Argentina too and I'll be asking these same questions in their forums too. Any advice is welcome! Thank you!!

Joey