In Beijing you will have to visit the Forbidden City, Temple of Heaven, Summer Palace, The Great Wall, Ming Dynasty Tombs, etc...
Three general areas you should research:
1. food -> In Beijing there are a lot more western style restaurants, however, you should be adventurous and taste the local foods (remember to drink lots of water and yogurt to keep yourself hydrated and your stomach fortified).
2. transportation -> City and Subway maps will be handy when you need to ask for directions.
3. hygiene -> Beijing has improved a lot in this area but it's still a "work in progress" - always carry toilet paper in case the public washrooms don't have any.