Getting organized, try to add "My Tags"

Hello, I am new, so please forgive me if I ask a stupid question.
I have library which contains f.e. 342 novels. I have transferred all of them to calibre but as I have plenty more books, which need to be in other categories, have another "My Tags"... I want to give these 342 books the tab "Belletristic".

How do I do that?

Thank you