I Would like to have a register form like that one http://www.blackfire.eu/account.php?act=register
so I thought I go with this tutorial https://www.youtube.com/watch?v=XSJYDjhkZJ0 but he doesn't show how to add fields or how to change some. So is some able to help me?
You can add customer specific fields by adding fields to the Billing or Shipping Customer profiles.
Via the Admin menu: Store -> Customer profiles -> Profile Types -> Billing/Shipping Information -> Manage Fields.
URL: yoursite.com/admin/commerce/customer-profiles/types/billing/fields
If you need to customise the checkout form further, there are a number of ways, including the following:
1) Use an existing Commerce module that provides extra fields or checkout "panes".
Commerce Agree Terms - Adds a Terms and Conditions checkbox
Commerce Extra Panes - Lets you load nodes within the checkout form
2) Create a custom module and add your own checkout pane. In this pane you can add any required fields. This tutorial provides further details on how this is implemented. http://atendesigngroup.com/blog/custom-commerce-checkout-panes
3) Use the standard Drupal hook_form_alter() to modify the existing checkout form. There is an example here https://drupal.stackexchange.com/questions/107310/how-to-alter-commerce-checkout-form-in-a-custom-module
