Multiples of 2, 3, and 5 cover most numbers. Just the way it is. I take it you haven’t done mush algebra, but that’s ok. the first link gave you many ways to check for primes.

As an exercise start counting and see if the number is evenly divisible by another number. 2 & 3 are prime.

4 is divisible by 2

5 is prime

6 is divisible by 2 and 3

7 is prime

8 is divisible by 2, 8 = 2 x 4, 4 = 2 x 2, so 8 = 2 x 2 x 2 (this is 8 factored. Factoring is the smallest possible numbers multiplied together to make a larger number. In this case 8)

9 = 3 x 3

10 = 2 x 5

11 is prime.

Continue for yourself.