I didn't start drawing (in general) with pixel art, and I think that probably made my pixel art journey a lot easier, because I was able to focus on pixel-level techniques on their own, without also having to learn to draw at the same time. I recommend it. If you struggle with visualising things, practice it in a medium you're already familiar with (or, lacking that, just any high-res medium where you don't have to worry about pixel-level polish, such as digital painting or pencil and paper). It's much easier to learn to draw in an "easy" medium where you can focus on just drawing.
As for learning to depict different things, it mostly comes down to drawing things a lot, and trying different ways. Do studies from life and photos, try drawing the same kinds of stuff in different styles and different contexts. You can also study others' art (not just pixel art!) to see how they stylised various things, and ask yourself what you think is effective about that style, what isn't, why they chose to deviate from reality in the ways that they did. Drawing well comes down to a lot of observation and questions, of reality and of fiction.
I know that's not the "how do I do the visuals for THIS game" answer you want, but coming up with a specific game's visuals comes from knowing how to draw things in general, and being able to select the appropriate look for the game's tone.
Before you worry about tiles, try to make some mockups of full scenes without worrying about tiling, so you learn to communicate the visuals in pixel art before you add the extra challenge of making them tile. Or, maybe even try making a mockup at a much larger resolution than your game, so that you can focus on just communicating your visuals, without the additional requirements of pixel art. In any case, starting with mockups, regardless of how you do them, is a good idea, as it allows you to work on whole scenes and make them work well. Creating tiles or objects in isolation tends to lead to mismatching scales and styles. It's common to find that things don't quite work together if you do everything separately, and starting from mockups helps avoid the extra re-doing.