To find the GCF of a number. You need at least two numbers. List the prime factors of each number and multiply the factors they have in common.
For an example we will use 12 and 20 for our numbers.
Now to get the prime factors.
12
2 * 6 = 12
2 is prime
3 * 2 = 6
3 is prime
2 is prime
20
2 * 10 = 20
2 is prime
2 * 5 = 10
2 is prime
5 is prime
The prime numbers that they have in common is 2.
So we multiply 2 * 2.
One from each number we are factoring.
GCF = 4