Be going to

     Be going to is used to predict future events that are very near (and can be seen) or seem sure to happen. Such predictions are based on present evidence.

Examples:
     –  My sister is going to have a baby.
     –  Look at the clouds; it‘s going to rain.

Will

     We can use ‘will’ (‘ll) to talk about the future and make future predictions based on personal opinions. For the negative, we can say ‘will not’ or ‘won’t ’.

     Structure: subject + will + verb (infinitive without “to”) + complement

Example: 
     –   I think scientists will discover a cure for cancer soon.

     We can also use the words “definitely”, “probably” and “certainly” to make emphasis.

Practice this lesson here: FUT001.