Understand the rules before you begin.

Before you get started on your video, view this video to learn the basics of following Copyright law.
YouTube Copyright School (4:30)
View this video to better understand how to use content licensed under creative commons.
What is Creative Commons? (3:19)

Use these resources along the way.

Images
Google Advanced Image Search Select "Free to Use or Share" next to usage rights. On the search results page it will say "Labeled for Noncommercial Reuse."
Flickr Flickr allows you to set a filter so that you only search creative commons licenses. Make sure you select this option on the home page.
Creative Commons Search Tool Use this prototype to find images that you can use and remix across several open archives.
18 Free Image Sites and Tools for Schools

Audio
YouTube Audio Library Browse and download free music for your project.
YouTube Music Policies This directory lists songs and their current policies set by the copyright holders
Freesound Freesound aims to create a huge collaborative database of audio snippets, samples, recordings, bleeps, ... released under Creative Commons licenses that allow their reuse.

Video
Videezy Free HD Stock Footage & 4K Videos!
Pixabay All images and videos on Pixabay are released free of copyrights under Creative Commons CC0. You may download, modify, distribute, and use them royalty free for anything you like, even in commercial applications. Attribution is not required.

Additional Resources
Best Practices for Attribution You can use CC-licensed materials as long as you follow the license conditions. One condition of all CC licenses is attribution. Here are some good (and not so good) examples of attribution.
A Copyright Friendly Toolkit Guidelines, categories, and tools to consider that will help you as you create, contribute to, and enrich our shared culture

Getting started with WeVideo

Sign up for WeVideo
WeVideo Tutorials: Projects (1:30)
WeVideo Tutorials: Dashboard Overview (1:44)
WeVideo Tutorial: Storyboard Editing Mode (1:22)

Customizing Your WeVideo

How do I add text to my video?
Learn how to add freeform text graphics anywhere in your video or edit predefined Theme text graphics that go with the Theme look and feel.

Rubric
Book Trailer Rubric