Briefly, we use will and be going interchangeably when we make predictions, like "What is the weather going to be like tomorrow?" = "What will the weather be like tomorrow?" There is no difference in meaning when we make predictions.
On the other hand, when we speak of things we plan to do (we have a prior intention to do something, or we have already decided to do something), we do not use will; we use be going. For example, "What is Christopher going to do when he graduates?" asks about Christopher's intention or plan.
Finally, when we decide on the spur of the moment to do something (like helping someone), we use will, but never be going. For example, if the telephone rings and you are closest to it, you may tell others in the room, "I'll get it."
Watch the video by RebeccaESL to review these concepts.