WILL vs BE GOING TO

WILL

We use it for:

• Predictions: 'In the future, the majority of people won't get married.'
  
• Offers and promises: 'I don't know how to use this computer' ' Ok, don't worry I'll help you.'
• Decisions made AT the moment of speaking: 'There' s a party tomorrow at Mike's' 'Really? I'll be there and I I will invite Susan'
• Future ideas based on opinions: (Generally they go with "I think", "I'm sure" "Iknow" "I hope"...) 'I think I won't pass the exam. It was too difiicult'

BE GOING TO

We use it for: '

• Plans and intentions: 'He's going to celebrate his birthday at Mambo Café this Friday, but he isn't going to invite his high school friends'
• Decisions made BEFORE the moment of speaking: There' s a party tomorrow at Mike's' 'Yes, I know it. I'm going with Susan'
• Future ideas based on evidence: 'Look at the clouds! It's going to rain'

If you need more grammar explanation, check this link:
http://www.angelfire.com/wi3/englishcorner/grammar/rules/future.html

Here, there are some exercises to practise!

http://englishgrammarsecrets.com/goingtoorwill/exercise4.html

http://englishgrammarsecrets.com/goingtoorwill/exercise5.html

http://www.better-english.com/grammar/willgo.htm