hypothesis/critical region help

So I have this word problem about herbs, scores on the test form a normal-shaped distribution with a mean of u=70 SD= 15 n=25 people take a herbal supplement everyday for 30 days, at the end each person takers a memory test and the mean score for the sample is calculated to be M=78.

Identify the critical region for the test assuming that the researcher is using =.05

I don't understand what the critical region is, how do I figure that out? Or how you know to use .05?? In this case it says too use .05, but what if it doesn't say?