Selections are really at the core of editing in Photoshop: they are what allow you to apply edits, effects, and more precisely and convincingly, while also enabling you to rearrange, copy, substitute, and remove elements of an image. In this great tutorial, you'll learn the most common tools and methods for making selections in Photoshop.
I'll be the first to admit that I can always stand to improve my selection abilities. They're definitely their own art, and much of the skill lies not only in choosing the correct tool for the job and being apt with said tool, but understanding how Photoshop "thinks" so you can anticipate and take advantage of how the program makes a selection or tweak one that has already been made. This great video from Anthony Morganti will walk you through the process and point out some of the options each tool has to make more precise choices. Whether you're making composites, getting rid of flyaways, or simple applying selective adjustments, the better this skill, the better your edits. It can be frustrating when you're first starting, but be patient and work on fine-tuning your skill, and you'll notice your overall edits benefiting from your newfound aptitude.