Adding Text To Empty Space on Left!

Hello All

Firstly, this is my first post so if this is really obvious i apologize i have searched for an answer but can’t seem to find one.

Using the default wordpress theme twenty fifteen, i have a side bar on the left (which I have reduced the size of) the primary content box and then empty space to the right.

I would like to put A Short header and a sentence in this section, as well as a picture?

Has anyone done this so far, and if so could they explain how.

I would ideally like this to appear on all pages and posts so I assume I would do it in the Custom CSS.