To find the square root of something, you have to find the number that can multiply by itself to get the desired number.
So: 12 x 12= 144
So this means that the square root of 144 is 12.
Next: 16 x 16= 256
So 16 is the square root of 256.
The square root of any number (not negative) can easily be found by inserting that number into a calculator, then hitting the square root button.