Cite your sources. Include a note that the text is taken from Wikipedia (or wherever) and include a link to the source page.
Make sure that the image you are using has a creative commons license that allows you to use it for noncomercial purposes. Usually there will be information at the image source about whether attribution is required. When in doubt, attribute.
Sure. I would try to avoid relying heavily on it. My suggetion on something like this would be to watch the video at a time when you are not coding. Pay attention to how they are describing their problem solving, instead of watching the code that they write. Then, when you come back to CodePen in an hour or a day or whatever those core lessons will have taught you, but you aren’t likely to (even accidentally) copy their code.