First you should draw yourself a square.Since you know it is a square you can find each side by dividing it by 4. 32/4 = 8. Since it is a square you know that both the length and width are 8. If you draw in the diagonal that is a right triangle. Use the Pythagorean theorem a^2 + b^2= c^2 to find the length of the diagonal.
8^2 + 8^2 = c^2