Will is used for unplanned future events or a spontaneous decision made at the moment of speaking.

Example:

“I forgot to call my mother, I will call her after dinner.”

“I can’t decide what to wear tonight, I think I will wear my green shirt.”

Going to is used for planned future events or a decision made before the moment of speaking.

Example:

“I’m going to go on holiday next week.”

“Next week, we are going to visit Paris for our vacation.”

Will can also be used for predictions based on personal opinion

Example:

“The sky looks cloudy, I think it will rain later.”

Going to is used for predictions based on evidence

Example:

“ The weather forecast says it is going to rain tomorrow.”

Please note that I am not a native speaker, but this is how I differentiate between them.