Photoshop (CC 2018) Essential Training — Basics with Julieanne Kost: I’ve only surveyed this LinkedIn Learning, but it looks like she covers everything you’re going to need to get up and running.
Overall Photo Correction
If you have access to Adobe Bridge software (CS5 and up), this tutorial on making a RAW pic better is good.
An excellent tutorial on how to adjust levels
You can also use the levels tool to balance your colors
Understanding layer masks in Photoshop
The five essential blend modes for photo editing (excellent series of tutorials)