Almost whole last month I spent on wondering how the project should be done in technical way. Everybody knows the worst thing, developer can do is to start coding without any sensible idea how project should be created. Mistake made in early project stage involves many unnecessary changes, that could have been avoided. But now I know what exactly I need to develop this. I created entity relationship diagram to help me understand the idea of storing data. This can be also helpful, when I would back to it in the future.

The first thing I need to do is editor-xtd plugin button in administrator panel. This provides very comfortable interface (it's using modal-button class) to tag content. You can download alfa version of it from here. Right now, it shows "Insert Tags" button, when we want to add tag to existing article

or "You have to save content before tagging" otherwise.

The reason of it is obvious: Joomla! can't predict what will be new article ID (it doesn't exist yet in DB), so it just can't tag it.
Inserting (i mean: storing in database) tags is the next step I want to follow. After that I will start to provide admin back-end interface to manage added tags.

Thank you and stay in touch!