User Registration Form with Sprout Forms I try to implement a User Registration Form with Sprout Forms for hours, but it doesn't work ... 

I have replaced the value for the action from "sproutForms/entries/saveEntry" to "users/saveUser".

How should I create the fields (username, password, email, first name, last name), that the user are stored.

In a further step, I will add custom fields for the users.
You don't need to use Sprout Forms to create users on your website. Take a look at the Craft documentation for an example.