If you don't have the owner's manual, you can usually find a PDF version online by searching for "2004 Ford Mustang owner's manual" (or a similar search). Look for diagrams of both the underhood fuse box and the interior fuse panel (often located under the dash).
The problem might not even be a blown fuse; it could be a faulty relay, a problem with the motor itself, or a wiring issue. Checking the fuses and relays is a good first step in troubleshooting, but if you're not comfortable working with car electronics, it's best to take it to a mechanic.