these addresses are known as Vanity addresses and are usually computed using a project called VanityGen https://bitcointalk.org/index.php?topic=25804.0 (although there are other projects too that perform similar acts).
the more the number of characters you want the more time it is going to take to find such an address. you can find many speed reports in that topic which could give you an idea of how hard it is to find one.