Depending on your ECU, there can be 2-4 Electric Load Signal pins, they take +12v on load. If there are lesser pins and more signals which you want to feed, you can use diodes. Engine should have some vacuum switch to raise the RPM for AC. I connected AC on mine 4efe with a vacuum valve a.k.a 'ac vaali kuppi' which would turn on with compressor and raise the rpm, dont remember if ECU was even involved in the process, though it would be better if it knows that AC is on.
If you plan to do on later, i will refer you to the post by Xulfiqar on that
