Using external footage that you may not have shot yourself is a surprisingly essential aspect of creating a video, especially when starting out, or discussing historical context. Unfortunately, this can be a bit of an ordeal as quite a few sites don't necessarily want you to. What you'll find below is some guidelines on tools that allow downloads from streaming sites, and a few groups that actively support it.
As you get into downloading and using these tools, it's also a good idea to check out this page on Fair Use guidelines, and just be aware at what the limitations are.
Youtube has just about everything you can imagine in terms of content, and ClipGrab is a fantastic program for downloading those videos.
Alternatively, if you are comfortable working in the command line, Homebrew has an extremely useful tool called Youtube-dl that can be used for pulling media from Youtube. This is a great solution for Mac and Linux users.
You may find that you need a certain type of footage to reinforce an argument you are making, and online footage banks are definitely worth exploring for this. Here are some options:
Option 1: Handbrake - Handbrake is a free and downloadable software program, and here is a Youtube video showing how to install and use the program:
Option 2: VLC - VLC is more widely known as a media player, but it also has functionality for ripping DVD's. Here is a Youtube video showing how to install and use the program for this purpose: