Welcome! I hope you find this stuff as fun as I do.
To avoid an essay I'll give you a quick 1 line critique of each of your works. To be fair I'm pretty new to this myself (a year or so I guess) but I've picked up a few ideas along the way.

Firstly, please use the "insert image" control (below bold) to insert your images. you can upload them to Imgur first, then get the URL by right-clicking and selecting "copy image URL".
So, really not bad at all for 3 days in! Good work. Here are some thoughts and ideas.
The ball: Nice squash and stretch, but it changes volume as it bounces, so it looks unnatural. As it's 2D, make sure the area is constant all the time, so if you squash in one dimension, stretch in the other and vice versa.
Smoke: Looks pretty good!

Scene: Lookin' good! Your light source is coming from left and above, but you're adding highlights to the underneath of some things.
Skull: Difficult to read and a bit flat. I happen to know this tutorial, I remember doing these things myself. Hang on...

There. I'd probably do these a lot differently now, but also, I'd been doing pixel art for a bit more than 3 days when I did them!

Click to expand the image.
It's easy to shade and apply light to just the edges of a piece when you first start out, but you want to think about the shape of the object in 3D and follow the curves with the light and dark areas. What will stick out? Where will it cast a shadow? Just shading the very edge can make something look flat but embossed.
Same with the face. It looks flat (although very readable.) Think about how the cheeks and nose will behave in the light and apply shadow and highlights. (I'd brighten up the face on my one if doing it now.)
Mug: Maye try giving the steam a curl? Your smoke animation might help? I'd like to see some bolder shading to define the cylindrical shape of the mug, myself.
Shovel: I like the 3D effect from the shading on the blade of this one, but I think you have too much outlining. You could try getting rid of the internal outlines, letting the shading and different materials imply boundaries.
Sword: Your blade and handle are nice and 3D, but the guard is the same shade both in and out of the light. And a different volume?
I hope you get something out of these ideas and I'm not coming across too negative! Keep it up!
