On my Blog index page (overview of posts), I see for every post the whole content. But I want a short intro text for each post. The image plus a couple of lines, not more.
where can I change that?
I used a nice plugin that works immediatly and without css code : advanced excerpt
you can choose how much you want to see 
Hi there
Hope you are having a good day and thank you for your question
You can change this behavior without a plugin 
from WordPress settings > reading you can set to display full post or short summary, also, please check this link about excerpts: What is Excerpt in WordPress? How to Add Excerpt in Posts
Thanks!
Colorlib Support Team