Different ways of talking or writing about future events in English have different meanings. Here are some examples:

PREDICTING future events:

-It will probably be difficult to get them to pay on time.
-Prices will/are going to go up if the exchange rate changes.
-By this time next year, our costs will have risen by 25%.
-If the exchange rates change, prices will/are sure to rise.
-We'll still be unloading the goods at 3p.m.
-Please don't call after 12:30, Mr. Carter will be having lunch then.

Describing a PLAN or ARRANGEMENTS:

-I'm visiting/I'll be visiting Tokyo in November.
-She's arriving/She'll be arriving this evening.

Saying what you INTEND to do:

-I'm going to chase them up if they haven't delivered by Friday.
-I'm going to have lunch early today.

Making a PROMISE:

-I'll let you know next week.
-I'll make sure Mrs. Hanson calls you back as soon as she's free.
-Mr. Brown will pick you up at the airport.

Referring to FACTS(timetables or schedules):

-My plane for Buenos Aires leaves at 14:35.
-When do you arrive in Mendoza?
-Next Friday is a public holiday in Argentina.

Talking about something that is going to happen VERY SOON:

-I'm just about to/just going to phone Sydney.
-They're just going to/just about to start the meeting.

It's sometimes more polite to use will be doing:

-When will you be calling Mr. Lee in Singapore?
-Will you be seeing Mr. Kwouk while you're in Hong Kong?

instead of: When will you...? or Are you going to...?