I2C bus and Arduino Uno
When using I2C you must remember to put a pull-up resistor (something like 3.3kohm or 4.7kohm will work) from each line to power. I2C has the ability to communicate to multiple devices on each line using device addresses, so you will have to check what the device address of the xbee is in it's data sheet. Here is the I2C reference page for Arduino which will come in handy: 

http://arduino.cc/en/reference/wire

Edit:

I just recalled that Xbee does not support I2C directly, but instead uses UART for communication. Simply use the Tx and Rx pins on both your Arduino and Xbee. No pullups are required for UART. Here is the UART arduino page:
http://arduino.cc/en/reference/serial