I wonder how do I create new items with the K2 without going into administration, just using the site like joomla does with the menu "submit an article."

If you use the k2 login or the k2 user module, and you set up k2 user groups with the proper permissions, and add users to those groups, there is a link for them to add new items when logged in

Own William, thank you. I was getting worried about it. Thank you, case settled