I'm new to hardware and Arduino, but here's how I would proceed. As Paul mentioned, you should start off slow. You should start with the very basics and build things in small parts.
I'm assuming you have no Arduino experience at all.
To get familiar with the Arduino and uploading sketches, I would start with the blink tutorial. It is located here: http://arduino.cc/en/Tutorial/Blink
That will give you the basics of how sketch works. I would suggest trying this exercise using the built in led. Then I would try it using an led and breadboard.
This will give you the basics of turning an led on and off. There is a problem with this sketch though since there is a delay that blocks the loop.
Next I would do the BlinkWithoutDelay tutorial: http://arduino.cc/en/Tutorial/BlinkWithoutDelay
This shows you how to do a blink without blocking your loop.
Next I would do the AnalogInOutSerial tutorial: http://arduino.cc/en/Tutorial/AnalogInOutSerial
This gives you some basics on how to control the brightness of an led, but it also shows you how to use the Serial console.
The serial console is useful for debugging.
Now if you're comfortable with the code and how everything works in 1-3, I would try the WebClient tutorialhttp://arduino.cc/en/Tutorial/WebClient
This shows you how to send a query to Google and read a response.
You will need to know how to do this for your project I think.
I would go through the rest of the Ethernet Library tutorials until you understand how they work:
They are listed at this link - http://arduino.cc/en/Tutorial/HomePage
See the ones under "Ethernet Library"
As for using HTML and PHP, I'm not sure that would be the best way to control a robot.
I guess it could work if you made the Arduino/Ethernet board act as the server. It wouldn't work well if the Arduino was the client.
If you're going to use an XBee, why do you need the Ethernet?