List what you want the robot to do.
List what it will do if 'X' sensor reads 'z'.
List what you want to control from the PC & joystick.
Draw what you want to GUI to look like.
Choose a programming language/IDE for the PC side.
Take each task and work on the code to get that piece working.
1- Motor speeds. start, stop, turn,
2- serial interface between robot and PC. Get this working with a hardwired connection (don't forget RS232 level shifter). The put in the XBees and get them working.
3- Through the serial link turn LEDs on/off. This will require you to design a protocol with commands.
4- What is the Servo for? Define it. Then write code to control the servo.
5- On the PC get the joystick interface working so your PC program can read the joystick position.
Define what the joystick will do and add to the serial protocol command set.
6- Get the camera working and displaying in the GUI.
Last is to put all the pieces together.