Will is used to make promises, predictions based on present circumstances, and decisions made at the time of speaking. It is also used to state facts about the future. Going to is used to talk about future events that have already been planned or arranged before the time of speaking, as well as predictions based on present plans or evidence. Examples are given such as "I will marry and have two kids" using will, and "In 4rd ESO, we are going to Sevilla" using going to since it was a planned trip.