I travelled by myself. I stayed in a hostel in Ipanema. June 26th-July 6th. I met wonderful people and had the time of my life. I went to the confederation cup match, lapa a few times (once at night), I did sugar loaf, Christ, copa beach, Tijuaca, botanical and had no problems. There was times things got a bit sketchy but I was never really scared. I'm a firm believer that the appearance on TV is much worse than the actual place. In Ipanema I felt very safe.
There is way too much information to list but a few things I ran into:
Iphone SIM card is hard to come by. iphones take micro cards, only regulars are sold but you can cut them down if your skilled.
Learn some Portuguese. Hi, Bye, good morning, how much?, numbers. Nothing is worse than not knowing how much something costs and wanting to buy it.
Make sure you have an international bank... I had a regional bank and ran into a few problems with getting in contact with them when i had a few problems.
Anyways if you have any specific question just let me know. Just thought I'd share my 2 cents.