Categories to top menu

Hello,
I don’t have much knowledge of programming and stuff and I have really struggled with this issue.
I’m using the theme Truly Minimal and I would like to add the “Categories” to the top bar as a drop down (instead of a side bar – because I have removed my side bar all together). I know how to add the categories one at the time to the top – but that’s not what I want. Preferably it would be called Categories and when you click on it it leads you nowhere unless you click on one of the categories added by me…
Is this possible – and if so then how? In a very simple way please ??
thank you so much in advance