What to look for in a motor?
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.


Here's how I would determine what kind of motor to look for:

First I would figure out what kind of speed I want my robot to achieve. Next I would determine roughly how much my robot weighs. I would do a quick survey of what motors are available keeping in mind the mechanical characteristics. A quick calculation of the max speed of a robot is:

$$speed_{robot} = diameter_{wheels} * \pi * speed_{motor} / gear~ratio
$$

The required torque is a trickier quantity to calculate as you'd need to know what kind of friction/resistance your robot is going to encounter and know roughly how much your robot weighs. Assuming rotational inertia of the wheels is small compared to the mass of the robot (and if I did my math right), the ideal speed of the robot vs. time is given by:

$$
V(t) = w_d \cdot \pi \cdot \omega_{NL} \cdot ( 1 - e ^ {\large{- \tau_S / (2  \pi \cdot m \cdot \omega_{NL}) t}})
$$

Where:

$$V(t) = robot~speed$$
$$w_d = wheel~diameter$$
$$\omega_{NL} = no-load~motor~speed~(including~gear~ratio)$$
$$\tau_S = stall~torque~(including~gear~ratio)$$
$$m = mass~of~robot$$
$$t = time$$
$$\pi = 3.14...$$
$$e = 2.72...$$

You can add the torque of all the drive motors together.

I would then design the electrical systems to fit the required electrical characteristics of the motor. This includes the motor driver, battery packs, and any wiring.

Gear ratio is the ratio between the input speed divided by the output speed of a gear box. They are used to speed up or more often slow down the output drive shaft speed. As a consequence of the speed ratio change the output torque also changes. In an ideal gear box with 100% efficiency the gear ratio is equal to the output torque divided by the input torque. This is mostly true for real gear boxes, but in reality most gear boxes lose ~2% between gear mesh (note that this relation is exponential as each gear mesh produces an input/output set, so for a gear box with 12 meshes the efficiency drops from 100% to ~78.5%).