In many cases it makes no difference which one you use. Tomorrow it's going to be cloudy. Tomorrow it'll be cloudy. We use "going to" for our intentions and "will" for things that are more certain. I'm going to major in economics (I intend to do that.). I'll finish my degree in 2018 (If I follow this plan, the degree will come because that fulfills the university's requirement.) We only use "will" if we make the decision or plan at the moment we speak, that is, we haven't planned it in the past. For example, if you see someone who drops a stack of books and want to help her pick them up, you say "I'll help you..." not "I"m not going to help you." "will" and especially "won't" are used to show that a person refuses to do something. "My son won't eat his vegetables. How can I make them more appealingly? These are some differences, but really there are many times you can use them interchangeably.

You use "will (verb)" when you make a sure prediction about the future, but without present evidence. It's just an idea. You use "be going to (verb)" when you make a prediction with present evidence. If the temperature is dropping and dark clouds are gathering, then you can say "it's going to rain" ("it will rain" sounds silly because the coming rain is very obvious). For yourself, use "I will" when you have the intention but no plan yet as to how to do it. Use "I'm going to" when you do have a plan in your head.

You use "going to" if you have the intention to do something, but it could take place or not.So you would be talking about plans for the future. You use "will" if something will certainly happen. Examples: I am going to see my grandma tomorrow. She will turn 80 and we want to celebrate. My dad is going to try a new diet next week. I will be surprised if he looses any weight.