In this situation we use "I am going to" and not "I will", because "I'm going to" is used for plans that were made in the past. You already knew you were going to work today, and you already knew you were going to tell your manager you were quitting. "I will" is more used for situations where you make a decision while you're speaking, or in the present. For example, you might say to your friend, "I'm going to the shop." You already decided to go to the shop, and you are letting your friend know. If your friend answers "OK, I'll come with you!", it means they made that decision while you two were speaking.

Are you working on talking with someone like this