but it probably wasn't the best place to post because nobody replied
It was probably because nobody knew.
What you are asking is very specific and basically you are saying "has anyone done this. if so then please tell me how".
Assuming from your previous lack of response nobody has.
Now what we are good at is general information or information about how to tackle researching into this project yourself.
I don't know where to start or what I'll need.
Well start with what those two things need to control them. I have not got a Gameboy Micro but you have to find out what sort of input it would take to control what you want. Does it have any form of control input it can switch to like MIDI or OSC? If not the project falls at this first hurdle. Unless the makers of the software have allowed any sort of external control then just forget it.
Once you find out this you can then think about the Ableton Live end. Can it take the same form of input or will it need another form of input? I know Ableton Live is quite flexible into what you can route into it.
Then you need to work out what sort of inputs you want to control it with, buttons, light beams, pressure sensors? Then think about getting an arduino to take these inputs and turn them into one or two control outputs (depending on if Ableton Live can take the same as the Game boy). Only then will we know what sort of Arduino would suite best and start to think about a schematic.