heating = add heat to something (it does not have to get "hot", just increase its temperature) warming = to heat something until it is warm (not hot) boiling = to keep water at 100 degrees C, or cause a liquid to change to a gas

They are all correct. The word you choose depends on how hot you want the milk to be. You might 'warm' milk for a baby, to say 40 degrees or so. You can 'heat' milk if you want it to be quite hot, to put in coffee. You would 'boil' milk you want it to bubble in a pan at 100 degrees.

I would say "I am heating a cup of milk" or "I am heating up a cup of milk". Any of those are fine, but I would only use boiling if the milk was actually being boiled. Hope this helps.