To find a GCF (greatest common factor) you need to find the largest number that both numbers are divisible by. I would first list all the numbers that 12 is divisible by:
1,2,3,4,6,12
Now check if 20 is divisible by each of those starting at 12
12 doesn't work so check 6
6 doesn't work so check 4
4 works so that is your answer
Hope this helps!
