Installing Forge on Windows 8.1 shouldn't be any different from installing on most other operating systems or versions.
The Windows installer for Forge is at http://files.minecraftforge.net/ (if a direct download link is more useful, here). Once you have the .exe file downloaded, run it, and follow all the directions. It will automatically install Forge for 1.8, and add a
mods folder to your .minecraft directory.
If you mean by "won't open up like it should" that the installer file doesn't run properly, you may want to make sure your browser or something else isn't blocking the file from downloading or running. After you've installed Forge, the only other thing you need to open is Minecraft. If Minecraft won't open, it could be because of a variety of things- you could have incompatible mods or versions, or some other problem altogether.
If you're being prompted to open something other than Minecraft or the installer, you might want to make sure you have a legitimate Forge download- again, here.