You first need to figure out which Win8 you have... 32bit or 64bit... once you know what you have, you install the version that is appropriate (you should actually have only downloaded the 32-or-64 bit version for your Win8)
If you have just purchased and downloaded Premiere Elements 12, I suspect that you did not purchase it direct from Adobe since Adobe is no longer selling Premiere Elements 12.
If you were purchasing from Adobe direct, I would have expected a Adobe Premiere Elements 12 Folder on your desktop. In that folder would be a PRE12 Folder and a ReadMe Folder.
You would open the PRE12 Folder and use the setup.exe file inside it to start the Premiere Elements 12 installation. In that scenario, the computer 32 or 64 bit matter would be behind the scenes as it were.
I am assuming that you have Windows 8 64 bit computer. In that case, open the PRE 64 Folder, find the setup.exe inside, and double click the setup.exe to start the installation. Important. Have you received a purchased serial number for Premiere Elements 12 from the seller? If not, do not bother continuing with the installation until you get the purchased serial number.
Best do these downloads and installs with the antivirus and firewall(s) of your computer disabled.
Please let us know if you have any questions or need clarification.