What data analysis test should I use?

I need a recommendation for a data analysis test I should for the following situation:

I have two independent variables, A and B. In this situation, every A value is linked to a B value. I want to determine if the value of A has an significant affect on the mean of B.

i.e. If A is a small value like 10 and its B value is a large value like 30, I want to determine if A being small has a significant impact on the mean of all B values. I want to see how small an A value needs to be to disregard it. The A value ranges from 1 to 1,000,000 and the B value ranges from -20 to 20.

Any advice? Thank you.