Itabira is a relatively small city in Minas Gerais (~125,000 inhabitants) but a city of this size is definitely not safe. I assume AFS arranges for a host family? You should follow their suggestions, not go outside without them initially (to learn what to pay attention to and what areas to avoid) and after that go out alone only during the day and to areas you know are relatively safe.
Itabira is home to a large mine from Vale (one of the larger companies/employers in Brazil) and as a result a portion of the city is well educated and relatively well off financially. However, large portions of the city population is poor and safety is an issue. Logistics and transport will also be totally different than what you are used to.
Traveling to Brazil and to Belo Horizonte is the least of your worries. It's safe and relatively easy. For getting to Itabira I would recommend the Vale passenger train; it leaves once each day in the morning from the BH central station (a bit seedy area). Get an executiva class ticket, which is safe and comfortable. vale.com/brasil/…default.aspx
This leaves you with getting from airport to the city. I think that in your situation the Connexão Aeroporto (executive bus to the city center) is the best bet; there will be several hotels near the drop off point. http://www.conexaoaeroporto.com.br/home
Going from bus drop off to hotel and from hotel to train station requires some form of taxi service. Uber is your best bet, imo. It is definitely safe. Get a "select" car for better service. It's worth paying a few Real extra for that.
You are in for a big experience! Minas Gerais is a beautiful state. We have visited a few wonderful natural areas this summer (santuário do Caraça and serra do cipo) and have really enjoyed it. The people are wonderful and the food is excellent, but Monte's warnings are important to take seriously! Good luck and have fun!