The three biggest “bang for your buck” processing concepts a landscape photographer should know are:
1) Layer masking allows for adjustments to be applied to some parts of a shot and not others. Plus it's how you can manually blend bracketed exposures (eg the image at the top is comprised of three blended exposures)
2) Colour correction is also extremely important because how often do the colours in an out-of-camera image actually look how you envisioned your finished shot?
3) Contrast boosting is the third piece of this critical puzzle, because this is how you bring out the colours and textures of your image to create a finished image that really stands out.
Of course with Photoshop being the incredible application that it is, we can dive infinitely deep into each one of these concepts, but before we do that, we need a starting point.
That's why the very first video in my Essentials course is a 1+ hour end to end walkthrough processing the photo at the top of this email.
It's a great introduction to these techniques and concepts, whilst at the same time being more than just theory because you're seeing their practical application on an actual image.