Adding and styling a css class for a wordpress post

I am new to wordpress development, and I am currently using the Customizr theme.

I have been searching for days on how exactly I am able to add a CSS class to a Wordpress post, and be able to style it.

I realize I am able to add in html for instance but I cant seem to add a class to it and subsequently adding style to that classname.

I know I may be lacking a lot of knowledge, is there a guide you guys can recommend so I may eventually learn how to do this? I have studied how to work with hooks and also briefly on how the loop works, but have yet to tested it out.

Thanks,