You can either use Devise's default controllers or add your own. Here is the link to the registrations controller:
https://github.com/plataformatec/devise/blob/master/app/controllers/devise/registrations_controller.rb
Keep in mind, Devise handles actions a little differently if you're planning on adding records to the User model.
You'll need to add something like this to the registration controller if you plan on adding something like a username:
before_filter :update_sanitized_params, if: :devise_controller?
def update_sanitized_params
devise_parameter_sanitizer.for(:sign_up) {|u| u.permit(:username,
:email,
:password,
:password_conf)}
end
You'll also need to add this to you routes.rb:
devise_for :users, :controllers => {:registrations => "users/registrations"}