What's the diference betwen "I'm going to do it tomorrow" and "I'm doing it tomorrow"
Ways to express the future:
1.Present Simple. When it is something you can find on a timetable, calendar or similar time-schedule.
Ex: - What time is your train? - It leaves at 7.30 p.m.
2.Present Continuous. When it is something that you have already made definite arrangements to do.
Ex: - When is your meeting with Mr. Thomas? - I'm seeing him tomorrow at 10 a.m.
3.Going to. When it is something that you intend to do and have already decided to do. When it is obvious that something is going to happen.
Ex: - You like Charles Dickens, don't you? - Yes, next year I'm going to read all his novels again.
Ex: - You look pale. Are you all right? - I'm going to be sick!
Ex: She is going to have a baby ( we can see that she is pregnant).
Ex: It's going to rain ( we can see the sky is covered in black clouds).
4.Future Simple. When you are making a spontaneous, immediate decision at the moment of speaking. When it is something you think will inevitably happen.
Ex: - Julia is back from her trip. - Oh, good. I'll give her a call.
Ex: - Goodbye. - Goodbye, I'll see you soon.
Ex: I think he will win this match.
5.Future Continuous. When you are thinking now about something you have arranged to do in the future.
Ex: - You must be looking forward to going. - Yes, this time next week I'll be spending all day on the beach.
As far as rules, I agree with Svet7777.
As far as a potential difference, try to translate this into your native language. What you will find out is that you would be able to express your sentence using at least two different ways, both of them will be correct and will have the same meaning.