Custom form Generation

I'd start here with the handbook section for creating modules:

http://drupal.org/node/231276

and the Forms API QuickStart guide:

http://api.drupal.org/api/drupal/developer--topics--forms_api.html/6

and look at the Form API reference:

http://api.drupal.org/api/drupal/developer--topics--forms_api_reference....

and also have a close look at hook_form_alter, which will let you alter the forms provided by other modules:

http://api.drupal.org/api/function/hook_form_alter

Note that there built-in features of Drupal core that help with features such as user login and registration, and there are many modules available to extend or replace these features. You should have a close look at the modules portion of the Drupal.org site to see if there are contributed modules that can help you.