Help. I'm old. I downloaded the files but I don't know what to do to start the game on my PC (windows 7).
It's a zip file. It's a type of file that packages multiple files into one to make it easy to download. You have to extract the game files from it first. Once you've done that, you can launch the game with a file called "Among Us.exe"
When the game gets an update, you'll have to do that again with the new version.
An easier way to manage the download of the game would be to download it through the itch.io app: https://itch.io/app
You can launch the game and update it through there, and you don't have to worry about zips or anything.
You should update to Windows 10 too, because Windows 7 lost support from Microsoft over a year ago now. It can be very insecure.