"Will" is more formal, and "going to" is more commonly used in conversation. When English speakers do use "will", they usually abbreviate it. For example, "I will do it" --> "I'll do it". Similarly, "going to" gets abbreviated to "gonna", even though this is very informal English :)

"Will" is more assertive and definitive than "going to". Telling someone that you "will" do something is more committing than "going to" but both have the relative same meaning. Example: "I'm going to do it" is more of a loose promise than "I will do it"

I'm sorry, but it's impossible to give a simple answer to this question, as the choice of future forms is often a very subtle distinction. It isn't true that 'will' is always more assertive than 'going to' - 'I'm going to' can often be extremely assertive. Nor is it true that it's a question of formality or that 'going to' is used more in spoken English - native speakers use 'will' or 'll' just as much in informal conversation as we use 'going to'. In fact, there is only one rule which usually works: 'Going to' is generally used for a plan, an intention, or something that you know or can tell is likely to happen. For example, you would look at a dark grey sky and say 'It's going to rain.' You would not use 'will' in this situation, because it's clear that this is going to happen. If you have made an appointment to see your boss tomorrow, you might say 'I'm going to speak to my boss tomorrow', because this is an arrangement that has been made - you wouldn't use 'will' in this situation. We often use 'will' for promises, offers, and decisions made at that moment. For example: 'This bag's really heavy' 'Don't worry, I'll carry it.' 'It's a secret.' 'I promise you - I won't tell anyone.' 'Sorry, we don't have any apple juice left'. 'OK, I'll have orange juice instead, then.' I hope that helps. This is a rule which you can usually rely on. But the best way is to listen to how native speakers use these forms and copy them.