In order to control a motor (or two), you will need a H bridge circuit to save yourself from a burnt pi, since the pi on its own doesnt supply enough current to drive motors or servos, or much else, really. A H bridge looks like this:
http://www.markallen.com/teaching/image ... bridge.gif
Where A and B are inputs from your pi. This will let you control direction of the motor using 2 inputs. While you can build yourself one of these circuits, an easier option is to get an integrated circuit which performs a similar function, such as the L293D IC...doing a google search of it will get you a datasheet, which shows the internal circuit. This IC can control 2 motors, using 4 inputs from the Raspi, which is useful if you want a car-like robot with a drive axle powered by one motor, and steering by another motor
A good tutorial on how to interact with DC motors is on the Adafruit learning system:
http://learn.adafruit.com/adafruit-rasp ... a-dc-motor
http://learn.adafruit.com/adafruits-ras ... ervo-motor
You will soon find that you will run out of pins on the rpi's GPIO set, and you'll probably need to get an external expansion board.
To have a mobile robot, you will need a battery power supply (for initial prototypes, where the robot isnt going to move far, you can connect it to mains via an adapter and a long cord):
http://www.ebay.com.au/itm/2600mAh-USB- ... 27d0708bc3
That one only supplies 1A max, and because its 2600mAh, doing the math, itl last about 2 hours at 1A, but thats in ideal conditions (assuming the seller's info is accurate). Since the Pi draws 700mA minimum, that leaves you 300mA to work with, which isnt really enough if you have things connected to your pi's USB ports....ANYWAY.
My reccommendation? Pull apart a toy RC car, and hi-jack its motors using the L293D chip. Chances are, you can use the car's power supply to power the IC.
Get this set up, so that your car can be programmed to move around on its own, have fun automating a 'robot' car first, then you can add in sensors which lets the car know where it is (ultrasonic sensor); you can then program in obstacle detection etc.
To access your raspi from your laptop or tablet, you can use SSH to tunnel in.
http://learn.adafruit.com/adafruits-ras ... -using-ssh
Hope I helped =)