I'll (I will) be there tomorrow.

I won't (will not) be there tomorrow.       

Will you be there tomorrow?

Ok -I'll answer it.

If the action is decided at the moment of speaking, we use will / will not (won't).

I'm sure he will help you.

When we say what we think or expect, we use will (won't).

I promise I'll help.

After promise we usually use will.

I'll call you when I come to my office.

When we refer to the future in adverbial clauses, we normally use present simple (after when, as soon as, until).

If you give me your address, I'll send you a postcard.

When we refer to the future in conditional clauses, we normally use the present simple.

I can't see you next week. I am returning/am going to return to Paris.

To show that the decision was made in the past, we use the present continuous or the 'going to' future.