There are a few primary specifications you should look for in a motor:
The type of the motor. There are a variety of different electric motors out there. In my experience the cheapest ones are brushed DC motors. These are fairly simple to drive: you send the motor a PWM signal and it spins faster with a higher duty cycle. Reverse the polarity and they spin the other way. I'm guessing this will probably fit your project the best.
Electrical characteristics. This includes the nominal voltage the motor runs at, what the stall current is, and what the rated current is (note: the rated current doesn't necessarily have to be greater than the stall current). Most DC motors I've used which are capable of driving semi-small to medium sized robots (about what size you have, though probably heavier) run in the 12-24V range so you may have to plan your battery packs accordingly. The stall currents has varied anywhere from a 1-2 amps to over 30 amps.
Mechanical characteristics. This includes the no-load speed of the motor, the stall torque of the motor, and any other mechanical characteristics such a rated torque or any gear ratio present (if in a geared motor). DC motors have a linear torque to speed ratio, i.e. they develop the maximum torque when stalled which decreases until the motor is spinning at it's maximum speed. The speed-power curve increases until the motor reaches half the no-load speed and then decreases. Typical DC motors have a very high no-load speed. Most I've seen vary between ~5000 rpm up to ~20000 rpm, though much high rpm ranges are definitely available.