How do you do those drop box thingies???

It's bad.
In a good way.

Neat ;)

This looks great. :D

Try to use different paint tools (eg. paintoolsai, firealpaca, opencanvas, etc). Paint.net is a really low-quality program (even i tried it, but remember it is my opinion), but all of the programs I just listed are very high-quality, and most of them have 30-day free trials!

The "Meh. x3" Looks like it's in pain O_o

They look good so far, but you could probably try to work on anatomy, form, and proportion (like in the last image). A site I like to use for practice is posemaniacs, which has many different positions and angles for drawing the human form. The only problem is that they don't have skin! Hahah, but then again that doesn't matter because you are just using it for form. I find that the best way to practice is by getting an image off the site, throwing it into Photoshop and tracing over it, taking note of the shapes. With enough practice you will be drawing anatomically correct people all the time! There are also various models here of different body parts that can also be used for reference.

Now, with shading it's a little more harder to practice. Being a photographer myself, I find it easier to shade things, because part of photography is taking note of the lighting! Now I don't have any sites that could help with that, because I'm skilled in shading, but I suggest analyzing other people's art. (I see that you have a Tumblr, as do I, and I know lots of people post art there, so it's not hard to find. You could probably also find various drawing guides, because I know people post them there). I mean, you don't need to go really into depth when analyzing someone's art, it could just be a few seconds making mental notes of techniques they use, real easy stuff like that. Also, downloading brushes (I know you can get them for Photoshop, but I don't know if you can get them for what program you are using) and using different brushes for texture and shading is also good.

One more thing, ALWAYS USE LAYERS. Don't forget that, or you'll be in trouble.