Sidebar with product promotion

Hello,
I want to add a sidebar where a product from our shop is visible.
http://prntscr.com/m3plfb
You can see on the picture, I alreday have a sidebar with our newsletter singup.

Unter that sidebar I want to add the new sidebar with a product.

Is there a documentation online you can link me?

Thank you
Elias

Okay I found the way via widget.
http://prntscr.com/m3q247

But is it possible to show only one product.
I can set the counter of showing products to 1.

But as soon as we will add a new product to the store,
the new product will be on the sidebar cuz its set
to “show the newest”.