Start to build a robot
You'll need:


Emergency switch (it should be easy to cut the power off, even for small robots)
A chassis (you've found one already)
Motors (Obviously)
Motor control (Probably a dual H-bridge, think if you need reverse or not.)
Power (powering on (single use) batteries may not be your best option)
Controller (Arduino Uno, or something smaller or faster)
Sensors (Can be anything, depending on the requirements)


A way to communicate with the robot is also nice, often wireless (bluetooth/wifi/443mhz).

And also, it will be nice if the robot fits some kind of purpose, but this could be added later.
You could think of the robot having to pick up and carry something, extinguish a flame etc.

I advise to check out one of the many tutorials on building a robot, they often have a detailed parts list (sometimes even shopping links).