Future forms

Be going to + infinitive
Use going to, NOT will/won't, when you have already decided to do something.
Future plans and intentions
My brother's going to adopt a child.
I'm not going to go to Barcelona next week.
Predictions
It's going to rain.
With the verb go you can leave out the infinitive. I'm going (to go) to New York.

Present continuous: be + verb + ing
You can usually use present continuous or going to for future plans/arragements.
Going to shows that you have made a decision. We're going to get married in the summer.
Present continuous emphasizes that you have made the arrangements. We're getting married on July 12th.
Future arrangements
They're meeting at 10:00.
She's leaving on Friday.

Will/shall + infinitive
Use will/won't for:
Instant decisions. I'll have the steak.
Promises. I won't tell anybody where you are.
Offers. I'll carry that bag for you. Shall I help you with your homework.
Suggestions. Shall we eat out tonight?
Use shall with I and We for offers and suggestions when they are in questions.
Use will or going to for predictions.