This suitcase is too heavy! I will/am going to help you carry it.

English Explanation:

'This suitcase is too heavy! I will help you carry it' uses 'will' to express an immediate offer or spontaneous decision to help, prompted by seeing the heavy suitcase. 'I am going to help you carry it' can also be used to express a decision made now, but 'will' is very common for spontaneous offers of help.