First of all, this is lovely for a first attempt ! However, pixel art is a very "unique" medium and there are a lot of things which will make your art look more natural.

• Lines : it's just like with other illustration. Except if it's partof the style, you usually want o have clean lines. What that means for pixel art is that mlst of the time, you want your lines to have a constant ratio (eg : down 2, left 1 or right 4, up 1 etc). In your case, because you are drawing a rounded object, this may be a bit trickier. I would advise goind for sections of strictly increasing or decreasing vertical/horizontal ratios to keep your lines from feeling broken.

• Palette. Now, once again, this depends on style, but when working with pixel art, you have a very limited resolution. And while it may be tempting to use all the colors at your disposal to render that soft apple, it often ends up muddying the shape and making it harder to read. (This isn't so much a problem with a simple object like an apple, but it gets more troublesome when drawing more detailed works). So i would advise working with 64 colors (mayyybe 128).

Two final nitpicks : the back of the dimple on the apple seems a bit too open (?) and the bounce light feels a bit high on the apple. It feels like it should be lower and the shadows might need a bit of a deepening...

Sorry for the wall of text. It might seem like a lot of negativity, but what you are starting of with is really good. (It took me a few years of botched works to get to the okay level i am at today. You are starting off way better and i bet you are going to progress much quicker. Keep going, i'd love to see more of your work out here)