This link will explain how layer masks work in Photoshop. I would recommend using layer masks as they can be very useful in many ways within your photo editing projects.
Photoshop comes with many different Adjustment Layers to help you change everything from contrast to colours. Take a look at this resource to help you learn about the different adjustment layers: design.tutsplus.com/series/the-ultimate-guide-to-adjustment-layers–psd-26067
Take a look at this fantastic tutorial from PSD Stack. It helps you learn about light direction, using masks and adding hair to a portrait and also some nice touch-ups to the eyes and lips.