Overall
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.
Downloadable PDF: Intro to Photoshop CS6 [It’s free and covers the basics.]
Photoshop Essentials (many tutorials, beginner to advanced, with pdfs)
Overall Photo Correction
If you have access to Adobe Bridge software (CS5 and up), this tutorial on making a RAW pic better is good.
Levels
An excellent tutorial on how to adjust levels
Color balance
You can also use the levels tool to balance your colors
Cropping
Cropping to a specific size
Cropping to a specific ratio (CS6 Tutorial)
Layer Masks
Understanding layer masks in Photoshop
Blending
The five essential blend modes for photo editing (excellent series of tutorials)
Retouching
Realistic portrait retouching
Dodging and burning to make your subjects pop