I'd think of them like this: If you are eating at the moment you are talking, you would say "I am eating" (Present continous). If you are about to eat, or know a time in the future you will eat, you would say "I am going to eat (in 5 minutes, on Tuesday, next year etc). If you know you are going to eat at some point, but not when, you would say "I will eat (in the future). You can say "I will eat in 5 minutes" or "I will eat next year", and these are fine, but if you just say "I am going to eat" without any time indication, the people you are talking to would assume you mean almost immediately (well, I would anyway!).

Be going to expresses certainty about a certain event. I'm going to Paris tomorrow. Will expresses a future uncertain event with no indication of when or time frame I will marry soon.

No, you can't always use "will". This is a huge mistake. If you take the time to look online, you will find PLENTY of examples and exercises which show you the difference between these grammar forms for future. This is pre-intermediate-level English... there is more to learn in the upper levels!

If you always used 'will' to talk about the future, people would understand you. However, most of the time it wouldn't be a 'correct' use, and it would be different from what native speakers would say.