The future tenses are difficult to use in English because we have a great number of options to choose from. In certain cases there is little or no difference in meaning between two or three tenses. There are also some very specific situations where only one tense can be used.

The main confusions comes from being too reliant on will as THE future tense. If you have decided to do something before the time of speaking, will cannot be used. Instead the present continuous or going to tense should be used.