Before we jump into where you can find grist for the movie-making mill, let's lay out what sort of gear you'll need to start making your own movies. You may own a PC or smartphone with basic audio and video recording capabilities, but all you really need is a computer with Internet access.
You can download plenty of free multimedia content online, including audio clips for your soundtrack and full-length videos you can cut and edit as you please. The lion's share of this free content is going to come from one of two sources: the public domain or the Creative Commons. The actually has an excellent that lets you find works with Creative Commons licenses across a variety of different websites, including YouTube, and the . The Creative Commons licenses allow creators to release their works for public use without giving up the other protections provided by copyright (for example, work with certain Creative Commons licenses can't be republished for profit) and as of this writing over 400 million works have been licensed under Creative Commons, so you have plenty of material to sort through.
If you want to be more specific you can also check out dedicated free media websites like the, which hosts a ton of music licensed under Creative Commons. If you need shorter audio clips (explosions, screams or other sound effects) check out , an online database of sound effects created by users and freely available for use under the Creative Commons license.
If you're more of a history buff you might consider grabbing some classic images and video clips for your project. Plenty of popular websites license their images for use under Creative Commons (including Wikipedia and ), and chances are you're already familiar with one of the best sources for free high-quality images: .