Many 2008 Wranglers didn't come with a cabin air filter from the factory. If you want one, you'll need to either install an aftermarket filter or have a mechanic do it. There are several aftermarket filter kits available that include instructions and everything needed for installation. The location for installation will vary slightly depending on the specific kit you choose, but it generally involves accessing it from behind the glovebox.
Before attempting installation, searching online for "2008 Jeep Wrangler cabin air filter installation" along with the specific kit you purchased will yield videos and written instructions.
