Basically, right now, you got a CNC machine, so 3 axis.
For a 3D printer, what you need is to add one more axis: the extruder.
So, you’ll have to purchase:
-An extruder motor, so not only the stepper motor, but the complete extruder assembly.
-a motor driver (pollolu stuff) to give power to this new motor
-Wires, to connect the thing
-A hot end assembly (hot end, thermistor, cooling fans, all the things)
-A glass plate, which will be your bed. No need for a heating bed at firs, you can always puchase that later.
Then, I suggest you to build a new Z axis, dedicated for 3D printing. It is way more practical than having a multi tool support in practice, so you just swap the Z axis from milling to printer and vice versa, whenever you need. Just make sure that you have quick connectors, to disconnect all the things easily.
You can use this hotend support I made, if you want to. It allows you to use a bed sensor later, when you’ll feel confident enough to use it:
https://www.v1engineering.com/forum/topic/my-mpcnc-made-in-china/page/4/#post-34540
Software wise, you’ll need to:
-define the number of steps per mm of your extruder motor. The same way you did it for your other axis motors
-define your thermistor type, the thermistor is the little sensor which tells the arduino what temperature your hotend actually is. Not difficult to do, everything is well explained in Marlin
-I think that is pretty much all, maybe some temperature alerts to disable, I forgot. You can always come back here to ask whenever you have a problem anyways 
Connect your thermistor to the appropriate pins on Ramps, then your heater cartridge, then your extruder motor and you should be good to go, there is nothing really complicated. The difficult part will come later, when you’ll have to dig into the fine tuning to make it print well.
Just get the thing started, you’ll see it’s no rocket science.