The better you are at selecting things, the better you are at Photoshop. It's one of the most fundamental skills. This great tutorial will teach you a multitude of ways to effectively and efficiently make selections in the venerable app.
The ability to make good selections is a crucial skill: whether one is swapping out a sky, making a composite, or applying adjustments to only part of an image, they arise in almost every scenario. Unfortunately, as smart as Photoshop is, it doesn't think like we do: where we see grass, it sees a swath of green pixels and edges. Nonetheless, it's pretty clever at handling that collection of green pixels, and as you begin to understand how it "thinks" versus what you need to do, you'll have a better idea of which tools and techniques are appropriate for each scenario. Be sure to stick around for the full tutorial and take heed of the last lesson on how to defringe selections and objects. As good as your selections might be, there's a good chance you'll need to defringe them as a final polishing step, and having that skill is what can really push them into stratospheric territory.