I'm new to Arduino programming and I'm just wondering what type of motor controller to use and/or how to control motors using external power. Any helpful tips?

Hi Aaron,
What size of motors are you trying to control?

If you're looking to build a simple robot to experiment with, try one of the ones described on our Wiki.

The motor would most likely be powered off a 12V battery.

What magnitude of current draw do you anticipate?

EDIT:
For example, both this motor and this motor are nominally 12 V motors, but the first one has a stall current of 5 A, while the second one has a stall current 133 A. Thus, you can use a smaller (and cheaper) motor controller with the first motor than what you would have to use for the second.

EDIT 2: It also depends on how you've designed the mechanism that the motor is driving: motors aren't usually designed to be stalled for long periods of time, so usually you want to use some type of mechanical advantage that allows the motor to run more efficiently. In this case, you would probably not require a motor controller to handle the stall current in continuous usage.

rcahoon is correct. Motor controllers have power ratings, just like motors. In order to make a proper 'bot', you have to match power ratings.