Drupal CCK Content validation?

I've made a CCK content type in Drupal. It accepts user name, id and DoB. The ID is a 10 digit Alphanumeric string eg. X123456789.

I want to create a module for validating this input so what do I do? Do i write php code for it and where do i save this php code?

For example I want to validate the ID. The name of the content type is field_member_details. So if i write another module for this content type, how do i validate it. (I don't want validation logic). Need to know the whole procedure for something like this.

Thank You