Hello
@fierofreak179 . Welcome to the forums and welcome to PC gaming. It might not be as simple as 1-2-3, but it's worthing. You're gonna love it

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Back to your question:
Assetto Corsa uses Steam as a distribution platform. Typically, the installation folder would be something like : C:\Program Files\STEAM\steamapps\common\assettocorsa
In order to install the mods you simply need to unpack the files you are downloading, and copy the content in the appropriate folder. For unpacking I can recommend you to use
7-zip, as this knows to treat all the major file types.
If you are installing cars, you should be copying them to \STEAM\steamapps\common\assettocorsa\content\cars. This is typically true when the folder you have just unpacked, is named as the car
You should always check the files once unpacked; some modders prefer to release the package with a more complete path, starting from folder "content". In this case, you must of course copy the package to the folder \STEAM\steamapps\common\assettocorsa.
If you are installing tracks, unpack and then copy the contents to \STEAM\steamapps\common\assettocorsa\content\tracks
Most of the mods have also a readme file included with the basic instruction for installation.
EDIT: ha ha, ninja'd by frickin' pixel dude
