Cannot Understand how to do simple multi tread.

Hi,
I need some help in solving this problem.

I have a form (formMain) with two buttons, 'Start' and 'Stop'.
The Start button start some calculations and file I/O.

I need the Stop button to react immediately when pressed.
At the moment the reactiion of the Stop button takes too long.

I have been reading about 'multi treads' and I think thats what i need.
So if I am correct I need to have a tread to check if the button has been pressed.

However I cannot relate the examples I found to my situation.
I Have become completely confused. Mainly because I am a beginner :ehh:

Can you please give me a hand?

Regards

Francesco