Adding css class to post and styling it

I am new to wordpress development, and I am using the Customizr Theme

I am trying to add a CSS class into my wordpress post in order to incorporate styles into it but I am having difficulty finding any tutorial or guide to help me understand how this is accomplished. I am able to use

post content

for instance but when I cant seem to use

post content

(with .postclass defined in style.css of my child theme)

I have studied briefly on hooks and filters. And read up on the wordpress loop.

Is there a guide or tutorial that you guys can recommend I read up on in order to get this achieved? It seems like such a simple thing to do but I cant figure it out.

Thanks,