I read a lot of posts about templating but I couldn't find the right way how to set the new template. First basic question: When I went into /templates/YOURJOOMLATEMPLATE/html I didn't find the com_k2 folder. (I have K2 installed). What I have to do, to make manually this folder? Or something went wrong with the installation. The template that I am using for my website is rt_hyperion_j15.The second question:I read that I have to copy and rename the "default" folder from /components/com_k2/templates/default into /templates/YOURJOOMLATEMPLATE/html/com_k2/templates. But if i only copy that and rename the default template what is going to change? I still get the same template. How can i change this one with another template? How to install another template that will appear in Blog category and still have the  rt_hyperion_j15 as the default template of the site?

First : yes, you have to create it manually - if you install k2 it is not able to know which JOOMLATEMPLATE is installed.

Second: yes rt_hyperion_j15 is still your JOOMLATEMPLATE. With this Step you just copy the K2 Template.
Search the forum for your Template questions there are a lot of discussions and solutions. Even take a look inside the Documentation to understand how the Templating works.

For the Beginning