Are you talking about wordpress.com or your own hosted WP site?
If your own hosted WP site, after logging into you WP, click on Themes under Appearance
on left side in your dashboard. Click on Install themes tab, click on upload sub-menu and browse for your
.zip file and upload. Then activate.
Installing theme is always the same process.
Code:
https://codex.wordpress.org/Using_Themes
Just play around with it. You can even set up wordpress locally on xampp or wampp.
I moved from blogspot to wordpress. It was kind of like I don't know what to do at first.
But as you just play around, you'll learn things. Search for tuts on google, YT.