Okay, let´s think you are at home watching a TV show, and then someone rings you, he asks what are you doing (right now)? I am watching a TV show. It´s some activity that you are doing at the moment of speaking.
But then, you have a plan to do something else, to fix a coffee for you. So, you answer on: but I will be fixing a coffee (soon). You could use will or going to such "soon to be done" (but in the exact time frame you are on - seconds, minutes) plans.
To a project that doesn´t involve this time frame, but then it´s soon to be done (that is, it will take some more time - perhaps days) then you will use TO + verb (not in the gerund form).
Not all verbs accept are like this, that explanation would be to the case in point - the verb [to] study.
I hope this clarifies things a bit more.