All right, here's how to use a sieve to find prime numbers.
First, you make a table of all the odd numbers from 1 to n^x (with n and x being any number, try to make them small), but put a two where the one would be. (Let's say that n=12 and x=4.) First, circle the 2 and mark out all the multiples of two. Then circle the next number (3) and mark out all of the multiples of three. Repeat for 5 and 7. Skip the 9, because it's been marked out, then go on to 11. Then, since 13 is the first number that is more then n, all the numbers that weren't crossed out and are less than n^2 are all prime numbers. Then you cross out all of the multiples of those primes, and then all of the unmarked numbers less than n^3 are primes. Repeat one more time, and you're done.