I have a question regarding the shot I attached. I tried taking a pic of this landscape, but no matter how I played with the iso, going as low as 400, the sky looks really grainy if zoomed even just a little bit. Also, the hill feels kinda out of focus even though it was what I was actually focusing on, as the camera wouldn't focus on the sky. What did I do blatantly wrong?
Blue skies are typically noisy with any camera, and at any ISO. It is due to the RGB filtration of the individual photosites/pixels.
With the common Bayer filter array 50% of the photosites are filtered to a green centric wavelength, and 25% are filtered to blue and red centric wavelengths (each). So when you photograph a blue sky only 25% of the photosites are optimal for recording the scene.
But note that I said "color centric" wavelengths; because there is typically some overlap (varies by camera). So even if the sky was a pure uniform blue, some green and red photosites may still collect enough light to generate a notable response.
What this means is that; a uniform blue sky can be very confusing for the demosaicing algorithm to work with and results in color noise (typically red/magenta IME). This is very similar to recording very dark scenes where random photosites collect enough light to generate a more significant response resulting in random color noise.
Your image does lack sharpness... my best guess is that it is due to camera movement due to working with low ISO's (longer exposure times). It is also likely at least partially due to atmospheric interference over long distances (moisture, haze, heat, etc).
Edit to add: The response curves are for the D300 sensor with the IR filter removed, and show a higher sensitivity in the IR range outside of visible light (above 740) than a standard camera would exhibit.
Since you're in digital, there are a lot of things you can do that aren't possible in film without a ton of work. One of the biggest is being able to easily edit photos together. It's only a little more work taking the pics. If the there's too little light or too much or both together, take composite shots.
On this shot, I'd take the image of everything but since it's the bright moon and low light, I'd shoot the moon alone, the sky alone with the moon, and then the ground with no sky. In photoshop, layer the pieces on top of the pic of everything.
Here is your pic with basic general color corrections (explanation of those corrections) and your sky blurred. Make a copy of the entire pic, use smart blur to get rid of the blocks then gaussian blur to make the sky smooth. Then cut out everything with soft edges so it blends and put it over the top of the pic.
Here's the blurred sky layer over a plain green layer