Rio de Janeiro is a vast and exciting city. You should certainly plan ahead for where to stay and what you might like to see. With something for all tastes the city could be the most attractive travel hub for other parts of Brazil and may well be your entry point with many options for flights into the country.
Places to Visit
Ipanema Beach- the city has a chic beachside spot here where you will find the hippsters of Rio hanging out.
Maracana Stadium- if you like sport not just football then the nation show you their love affair with the beautiful game.
Sugar Loaf Mountain
Lapa- If you are the party goer then take some measures to find some to go with and enjoy the party.
Santa Teresa Neighbourhood
Corcovado and the Cristo De Redento Statue- One of the globes most recognisable landmarks Christ the Redeemer.
Jardim Botanico (Botanical Garden)
Buy some Haivanas! They are much better value bought from Brazil.
To stay safe, keep your wits about you. Don't go through large crowds / parties / clubs with your valuables in places they could be stolen from. Especially in street parties do NOT have anything in your pockets if you can help it! Even with say your wallet in there and your hand in your pocket it is quite possible that you will have multiple people pickpocket you forcefully, with one removing your hand and while that person gets your aggressive attention another will take your wallet. Ah what else. Don't go down to the beach at night beyond the bright street lights (a couple of friends were robbed at gunpoint down there on Ipanema Beach), don't buy drugs from people you don't know, and don't go into Favelas alone just wandering. Apart from that, just have fun! When people I knew got into trouble it was generally an easily avoidable situation that they just didn't think about. Lastly, LEARN SOME PORTUGUESE! I think it is both rude and a bit ridiculus to go there knowing none at all- you of course sign language always works. Meeting travellers there who didn't understand words like "Peligroso" meaning dangerous both in written and verbal forms worried me.
If you are keen to learn more then take a look at the Rio City Guide.