The simplest way to remember this is:

I THINK I WILL (you are not sure about it)

I KNOW I AM GOING TO (you are sure about it)

 

 

Snap (very fast) decisions:

I WILL OPEN THE DOOR.

I WILL TAKE THE PHONE.

 

SHALL IS ONLY USED IN QUESTIONS WITH I AND WE:

Shall I help you tomorrow?

Shall we go to the party together next week?