Hi toxic
I am assuming the pairing will be between corresponding values in the 2 sets for each subject. For example, Subject 1's first value in Set 1 (lets call it x1) is paired with Subject 1's first value in Set 2 (x2). Because you are pairing you need to calculate the difference between the pairs (x1 - x2)and will then work out the mean of your 25 calculated values.
If you had been doing a straight t-test you would calculate the two means first, but not when you are pairing.
Good luck