Present continuous - future arrangements; often with time, date, or place --> I am having a birthday party on the 6th.
Be going to + infinitive - future intentions, plans = already decided to do smth & you tell people about it (think - NY's resolutions)
--> I am going to do Meatless Mondays.
WILL - spontaneous decisions, predictions
--> I think I'll have a coffee. Don't worry, I'll pay. Tom will help you.