What are some Photoshop/Illustrator skills that you think a first-timer needs to learn?

The title says it all. I'm trying to get into using Photoshop/Illustrator but I don't know where to begin. Some advice would be much appreciated. I've been to tutorial websites like this, this, and this, but I don't know what to start with.

Some things I would like to learn are how to create renders like this or make some Feltron-style graphs for school.

Something more urgent is how to create the red translucent effect in this image.

Thanks in advance! (Sorry for the lack of words, I'm quite tired :( )