Hello
The easiest way to do this is to use the "Software Serial Library". http://arduino.cc/en/Reference/SoftwareSerial
With this library you can use other pins than 0 and 1 for serial communication. You could use the standard 0 and 1 pins for the communication with the computer and pin 2, 3 (just an example) for the Xbees TX and RX lines. The only problem you have is your shield, maybe you can't change the Xbees RX and TX pins. If you use the sparkfun Xbee shield you can simply switch the little switch to "DLine" and the RX and TX lines are on pin 2 and 3. If you use other shields like the original Arduino one, you may have to modify it a little bit.