So, I use this theme "Local Business" (Free)
http://wp-test.tuneld.com/
Here is my website.

I want to make that picture inside the white "box" to be a automatic slider..

"Choose your image or video. Optimal size is 363px wide and height 285px"
So, can I somehow by video or image make it a slider? Or can I somehow change it in CSS or smth like that?

I'm a beginner and have no clue...

http://cube3x.com/2013/08/30-free-jquery-slider-plugins/

pick one, and follow the instructions on how to use it

there's a great wordpress plugin called revolution slider. Very easy to use and simple to edit/skin. I use it for almost all my customers that want a Slider.