Many things you find online are copyrighted. While there are allowances in copyright law for using the material for educational purposes, for using short pieces, and most likely your projects will not create enough of a ripple on the Internet to be noticed, it is good practice to respect copyrights. After all, one day when you are famous you will want kids respecting your copyrights (and staying off your lawn!).
Luckily, there are plenty of options out there for images, videos, and music that you can use responsibly.
The Walt Disney Corporation is known for being very protective of their copyrights. (Fun fact: once upon a time*, the Kutztown School District received a letter from Disney lawyers requiring us to remove Disney images from a teacher's website.)
Creative Commons licenses are an easy way for people to set the rules for how others can use their works. If you find an image, song, or video with a CC icon or text, you can use it responsibly in your work. You can also label your own work with CC licenses and tell other people how they may use it.
This video explains how it works. And yes, he said zed.
CC Share-Alike by Creative Commons Kiwi
The two options that will matter most to you are:
BY - you need to give credit to the creator when you use their work
SA - You need to also let people use your creation in the same way.
The ND option only matters if you are going to edit the original work. The image behind the titles to my teacher webpage is a CC-licensed image I found and altered. Because of that, I found an image that was not labeled ND.
The NC option means you cannot make money from your creation. That seems... unlikely.
* Bonus Fun Fact: Disney does not own a copyright on the phrase "Once upon a time." And no, the teacher was not me.