Hi!

My goal is to allow users post items that contain only text. So I want to remove all unnecessary tabs in front-end editor - like categories, images and so on.
For most of this additional options I can just comment the relevant code in itemform.php file (or using css and "display:none")
But I can't get rid of some important options, like title or category.
Can someone help me with this?

(my thought is to state in itemform.php that title will always be "X" and category always be "Y" and use CSS to hide this options from users. My coding skills are really poor so sorry if my thought are idiotic)

There are already options to hide frontend editor tabs in the K2 options. Check the frontend options tab and hide the tabs you dont want display.

Other fields must be hidden trouch the template file as you mention.

Can someone tell me what should I write in itemform.php to set "title" to some default value "X" and set "category" to the only available category "y"? After that I will hide this from user using CSS

You can use jQuery's val() function api.jquery.com/val/
example: stackoverflow.com/questions/6041294/jquery-change-value-on-input