Sorry, I'm new on WORDPRESS. I am trying ATAHUALPA because this ist the most flexible theme I've tested.

My webpage is: http://alterssicherung.org/

The Title ist "Alterssicherung" - old-age assurance

There is a subtitle (second line) defined using other themes. But I cannot see where to organize to see the subtitle.

Tx in advance

I've changed the theme to have a better view. But it should not bei unpossible to answer because a lot of blogs do have a subtitle.

Are you talking about the 'tag' line (dashboard->settings->general->tag line)?
That should show automatically if you have the theme setting ATO->Configure Header Area->Show Blog Tagline? Set to YES