It looks pretty confusing when you look at the g-codes but the software I use actualy does all the hard stuff.
I start by making a 2d drawing in either autocad or maodelcad, just a basic line drawing thats in scale. I then take the dxf drawing into the software from www.deskcnc.com and chose the items I want to machine and thru the conversational dialoug boxes I choose the cutting tool, depth of cut per pass, total depth and finish pass that suits the material I am working with and the software generates the g-code(x-y-z cordinates) to controll the milling machine. The software is free to download if you want to give it a try, it will also generate 3d gcode files from a number of diff drawing file formats like stl, point cloud, IGS. Another good software package that will generate gcodes and also control a cnc machine (and its free) is kcam from www.kellyware.com
As far as the machine its fairly simple to attach some servo or stepper motors to the lead screws of your milling machine. Ususaly some sort of gear or belt reduction is needed, I like the gates xl timing belts from www.mcmaster.com, they also have the cogs, ballscrews and nuts as well as about anything else you can think of. Once youre motors are mounted and attached to the lead screws with a solid system like the timing belts you will need an amplifier/drive for each of the axis you plan to drive. The amplifier just takes the low voltage step and direction output from your computer , along with the output side of a dc power supply suitable for driving your motors and sends the appropriate signal to the motors to move the exact amount the software specifies. I like the amplifiers from http://www.geckodrive.com/ , there rated up to 20 amps and 80 volts. Most of the stuff in my shop uses 24v 10 amp power supplies built with a transforrmer, capacitor and rectifier. Nothing fancy. The drives from Gecko are available for either servo or stepper motors. The gecko drives will except standard step and direction commands from any of the 2 dozen software packages out there, a bunch of which are free. I like the software and controller from deskcnc, as it does 4 axis machining, thread milling, digitizing, and has all the industry standard cycle start, repeat and pause features but if your not using it every day the software from kellyware or mach 5 is great.
The difference in servos and steppers, stepper motors have multiple windings , each step represents some number of degrees of revolution, to move a given distance the software sends a step and direction command to the amplifier/drive and it in turn sends the proper number of high voltage/current pulses to the motor to move the specified distance. Servo motors are just big brush type DC motors but have encoders on one end of the main shaft, the encoders are usualy a plastic disk with hundreds-thousands of small lines and an optical arangement that can count the lines as they pass. The computer sends the same step and direction command to the drive, but in this case the drive applies current to the windings until the proper number of lines on the disk have passed the optical reader in the encoder. The advantage of servo motors is the motor cant skip any steps, the drive keeps applying current until the proper count is reached, the stepper drives simply send the specified number of pulses to the motor and dont have any method of ensuring the motor has moved the specified distance. This is why I prefer servo motors, steppers are usualy cheaper but not when you get into the large ones needed for full size machine tools. Alot of times good deals on servo motrs can be had on ebay, just make sure you get single phase , dc ,brush type. The 3 phase ac brushless motors can be very cheap but the drives are not so I would avoid those.
Every thing you need to get started is fairly cheap, the belt reduction system and mounts for your machine are hard to price without knowing model. Servo motors for a machine the size of the x3 sieg can be had around $100 each, the drives are about $115 each. For a bridgeport size machine motors can be had as little as $200 each, same drives as above. A simple dc power supply can usualy be put together for $75 or so.
Hope some of this rambling helps.