A number n bigger than one is prime if its only positive divisors are 1 and itself. Thus the first few prime numbers are 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, and 31. The only even prime number is 2 because any other even number has 2 as a divisor. We don't count 1 as a prime number.
Suppose n is a prime number. What numbers are relatively prime to n? Since n's only divisor bigger than 1 is n itself, any number not divisible by n is relatively prime to n. The nonnegative integers that are divisible by n are 0, n, 2n, 3n, and so on.