For 8/12 the simpliest way to look at it would be, they are both even numbers so obviously both number can be divided by 2.
8 divided by 2 = 4
12 divided by 2 = 6
Again you have even numbers so divided again by 2
4 divided by 2 = 2
6 divided by 2 = 3
2/3 cannot be reduced by any number evenly (the same number cannot go into both numbers). So your answer is 2/3.