Overview
Photoshop, it's important to note, is not intended for creating assets. If you're looking to create a logo, design a character, or make digital art, you're better off learning Illustrator. This is because Photoshop is raster based while Illustrator is vector based. Vector based programs display lines and shapes based on equations, which means that they are infinitely scalable. Raster based programs display lines and shapes based on saved pixels at a limited resolution. If you were creating a logo, for example, you'd want to do so in Illustrator because that logo could be printed at any size without any loss of quality, while a Photoshop image could not.
Photoshop excels when it comes to making adjustments to existing images. These images already have a set resolution, so you lose nothing by brining them into Photoshop. Whether you want to apply gorgeous corrections to your photographs or make your friend look like he's wearing a fanny pack, Photoshop is the tool for you.
Basic Adjustments
For the photographer, Photoshop is most useful for the adjustments it can make to your images. Most of these tools can be found under Image > Adjustments. If you aren't familiar with the basics of color theory, you should read this guide before getting started.
Photoshop also has "Auto Tone", "Auto Contrast", and "Auto Color" options under Image. Pros would never use these options, but they can be useful for getting an idea of what your image could look like. For amateurs, they may be the only three options you need to get a decent-looking image.
Selection
Most of the adjustments you want to make probably aren't for the entire image. Sometimes your eyes, and only your eyes, need to be a bit more saturated for that tinder photo. Photoshop offers many ways to select specific portions of your image. We recommend using a combination of the Quick Selection Tool (W) and the Polygonal Lasso Tool (L) for creating simple but effective selections.
Start with the Quick Selection Tool and attempt to get a rough outline of your final selection. This tool does its best to guess which pixels you want to select and which ones you don't, so keep in mind that it's far from perfect. You can adjust the size of the selector in the top left menu. You can also choose whether you are adding to your selection or subtracting from it using the icons in that same area. This is great for correcting the Quick Selection Tool if it goes overboard.
Once you have a rough selection, switch over to the Polygonal Lasso Tool. Using the same add/subtract toggles in the top left, you can use this variant of the Lasso Tool to add and subtract from your rough selection. Once you are mostly happy with your selection, open the Refine Edge window in the top left of the screen. This window gives you a lot of options to tweak your selection. Ticking on "Smart Radius" and increasing the Radius slider can be a great option for selecting hair out from against a background. The Smooth and Feather sliders will both soften the edges of your selection, but doing so can ruin the straight lines you do want. Use either in moderation.
Layers
Those familiar with the history of animation will recognize the image below as a diagram of the multiplane camera. One painted pane of glass is placed over another and another in a machine to create an illusion of camera movement. Layers behave in much the same way, though they aren't used to create motion.
Layers are simply the various images you have in your project in the order in which they appear. If you have a blue sky layer on top of your cut-out-of-a-boy layer, you'll only see the sky, but if you put the sky beneath the other layer, you'll see the boy against the blue sky.
Layers are controlled in the bottom right window and are essential to any kind of collage or advanced doctoring of an image.