"Will" is used when expressing a voluntary intention or a promise: "As a friend I will help you whenever you're in need" "Be Going to" is used when you've got something already planned and hence about to happen. "I am going to visit Japan on January" (you've already booked your flight) You can both use them in predictions: "I think he will win the competition" or "I think he is going to win the competition"

As expressions of intention, they both have the same meaning. "I will answer your question." "I'm going to answer your question." "The government will increase taxes next year." "The government is going to increase taxes next year." "Going to" is, perhaps, more casual and familiar.

"be going to" is used to express a near future (I´m going to do it tomorrow), "will" is used to express a more distant future (I will do it next week).