I'm new to Joomla and have no clue how/where to do this.
I made a slideshow with K2, but with all the position possibilities that are already on the website I can get it on the right place.
The slideshow should be under the main title of the page. I added a new position named 'position-slideshow' in the settings page of my slideshow. But where and what do I have to do to make sure this position = under the main title (which is a h1 tag).

Hello,

This is related to the template you are using.
Since I have no insight on your template and your overrides, I think you should address this issue to the template's developer.