There is no way to give a complete answer to such a broad question. Here are the highlights. Work out exactly what kind of social network you're trying to create. List out what you want your users to be able to do. What kind of privacy/security issues do you need to address? Once you've got all that you can go about finding/figuring out which contrib modules perform the functions you need. If there are no contrib modules that meet your needs you'd have to build a custom module for the function. These are broad answers to a very broad question.
Drupal SN success story is a good article to build social network with Drupal, but if you want make a complex social network like brazencareerist.com or popsugar.com, you should build the custom modules.
It's not that simple if you aren't a programmer. We hope that drupal 7 will be a large improvement towards this goal .
Acquia recently released an install profile for a business social networks that's pretty comprehensive out of the box. Very easy to setup and then you can customize... pm me if you need professional help. http://drupal.org/project/commons Ron custom drupal website