Hot to create a new module from scratch

Hi,

I have working experience with drupal, also used few modules(installation only). Never customized anything related to drupal. Now, I am looking to builde a module which need to have a better understanding of rhow to develop modules for drupal. I am looking for information which can help me to get a better idea to develop a module.
here is what i am going to do with that module:

1. When a user comes to our Drupal site and requests to set up an account we require them to enter a code that was given to them by Admin. (Admin would have entered that code into the database table).

2. Once the user clicks submit and Drupal begins processing we take the code that the user has entered and look it up against into the table.

a. If the code exists we set up an account for them in Drupal and with permissions that will vary based on the code the user entered. If they entered a code from

i. codeA - they will become a user of type codeA and be able to access the restricted section and get a special link in the email that is sent out to them.

ii. codeB - they will become a user of type codeB and be able to access the restricted section.

b. If the code does not exist we reject them. They cannot set up an account without a code.

Any help would be appreciated.

Thanks in advance,
Ramesh