How do I install a downloaded theme?

I’m new to web hosting, and now am trying out this wordpress.org. Hv downloaded a few themes, all now in my comp drives, how do I install them to show on my webpage? Sorry if this has been answered before, Maybe you could direct me to the answer/links. I’m going bonkers deciphering all these techie stuff myself – passwords for plugins, for codex, for themes… How many passwords are them anyway for just one wordpress.org web?