I've installed K2 on a Joomla 3.4.4 website and imported all the content. I also created a test 'item'. Any changes I make in the K2 component do not show on the website (i.e. publishing items, item edits, featuring). Do I need to disable Joomla article system before I can start editing in K2 or something else?

Have you change the menu items to point to K2 articles or categories ? Or have you publish a K2 content module anywhere?

These videos will help you get started getk2.org/documentation/k2class2012

I still needed to associate the menu items with K2, and thanks :)

Yes, Menus work the same as with regular Joomla! content.
You need to map your new content into menu items.