HOWTO: provide log-in (uid and pass) to Civicrm Contact - NEW or Existing
There are several ways to go about this.

I wrote a somewhat convoluted guide a couple of years ago for creating a Drupal contact for a bunch of existing Civi contacts:
http://wiki.civicrm.org/confluence/display/CRMDOC/Creating+a+Drupal+user+for+every+CiviCRM+contact

However, I'd say the best practice today is to use the civicrm_entity Drupal module in combination with the rules module to automate this.  There's an excellent blog post about that here: https://civicrm.org/blogs/petednz/creating-drupal-users-made-easy-single-or-multiple-contacts-civi-imports-and-webforms