The install folder for the majority of mods is in the assetto corsa steam folder - typically
C:\Program Files (x86)\Steam\steamapps\common\assettocorsa\
From there, if the zip file contains folder names matching the ones in that folder - that is, apps, content, and sometimes system or extension - you extract the zip file in that folder.
If it contains just a direct car or track folder name, then you go to content\cars or content\tracks and extract it there. Most mod download pages will tell you specifically what to do.
One common newbie mistake is to go to the assetto corsa Documents folder - in C:\Users\you\Documents\Assetto Corsa\. You cannot install mods here, the folders are for storing user-specific customizations (driver position, ffb strength, setups)