In spoken English, "going to" is the form used most often to talk about the future.

=> Use "going to" to talk about something planned for the future:
▶ I'm going to buy a new car this year.
▶ Is your daughter going to attend Dartmouth or Columbia?
▶ What are you going to do on New Year's Eve?

=> Use "going to" to make a prediction based on evidence you have now:
▶ Be careful. You're going to spill your drink!
▶ This plan is too complicated. It's not going to work.

"Will" is also often used to speak about the future. Use will (or 'II) in these situations:

=> To talk about a decision made at the moment of speaking.
Once you've made the decision, use "going to" to talk about it:
▶ I'll make the dinner reservations for Saturday night. Bob, please let Sara know that I'm going to make the dinner reservations.
▶ Mom: "If you don't go to bed right now, I'm not taking you to the zoo tomorrow morning!"
    Zach: "I'll go to bed now! Dad, I'm going to go to bed now."

=> To talk about things we believe to be true about the future:
▶ I'm sure you'll like your new job.
▶ The dean will serve for five years.
▶ Our new toaster will arrive next Monday.

=> To make a promise or an agreement with someone:
▶ I'll call you when I get to Paris.
▶ I'll send you the check tomorrow.

Note: Do not use "will" or "going to" in future time clauses.
SAY: As soon as you finish the report, call me and we'll review it.
NOT: As soon as you will finish the report, call me and we'll review it.