Soccer Robot is a very complex machine. It will require many different kind of sensors (like image sensor, ultrasonic range finder, infrared proximity detectors etc.) to follow the soccer ball. A huge microprocessor based circuitry with hardcored programming will make it work perfectly.
So if you are a newbie in electronics and microcontrollers/microprocessors this will be a bit difficult to build.
na not always i saw a soccor robot in J-car's catalog and it was just a simple remote controlled tittle robot with a catcher at the front.
may be you can just make a few remote controled robots and put in some solenoid to "kick" the ball.
its pretty hard to make an auto one and too exensive. plus if u realy want to play soccer with them u need more than one. I think a good idea is to make one auto and the other remote so u can vs the autobot at a match of soccer.