To adjust the voltage supplied to the actuator I assume I could use some sort of digital PWM, but I'm confused about the differences between analog and digital PWM. Is digital PWM just a matter of switching between 0 and 5V more quickly than you would if you wanted an analog signal? Also, given that the max DC voltage of the actuator is 12V, I was considering implanting a simple non-inverting amplifier between the Arduino and the actuator. I'm worried about overdrawing current from the Arduino as well, so I thought that implanting the non-inverting amplifier might help with that respect, drawing current from Vcc+ (please correct me if I am mistaken).
An analog signal suggests a voltage somewhere between 0 and V+. A digital signal is 0 OR V+. With PWM you're switching between 0 and V+ at a fast rate so that the average
voltage is somewhere between 0 and V+; it would be similar to flicking on and off a light switch really quickly in an attempt to make the light less bright.
To drive a DC motor bidirectionally you need an H Bridge ("Full bridge"). You can drive the motor in one direction using a transistor (or mosfet), but to drive it in both directions you would need an arrangement of four transistors to form a full H bridge.
Specifically to your Firgelli L12-S you should buy the identically priced L12-R instead. It has an RC interface and would be driven just like an RC servo -- no additional motor driver / bridge needed. Example: http://playground.arduino.cc/Learning/SingleServoExample
-- but you can find many more examples with a little searching.