I love your attitude, scarecrow.

Here are some general pointers.
Work on a neutral background. This means low saturation medium luminosity colors. It helps your eye evaluate the colors better than if the background was an overwhelming white or black. The grayish background of the forum is a nice color, so transparent backgrounds will definitely be okay, but you can pick green or red or blue hues as you work, whatever you like.
For practicing forms, don't worry about outline. Outline is stylistic and symbolic of border, meant to increase readability and separate graphical elements. Think in 3D and practice like a painter!
Next is the actual shading. There's something called pillow shading that crops up in a lot of beginner work because people don't work with actual cast shadow, only form bending shadow. Given, that usually looks horrible in pixel art because most people treat it as a thick gradient with incompatible colors. It winds up looking something like this:

Then there's banding. Simply put, it's just bands of sequential tones, much like pillow shading except it's more commonly seen on small areas like outlines. You can see it in action at the very top of your rock on the outline. It doesn't help anything but to clutter the image with unnecessary buffers. (but I don't think you need to worry about that at all right now, just keep that in mind for future reference)
You can get away with two or three colors. Dark, medium, and light. Also keep in mind that a lot of things don't have highlights. Here's a rectangular prism in a method that I've seen help people the most. See how it's clear where the light is and isn't hitting?

Practice the most basic shapes, thinking of them as a collection of planes. Then when you need to, you can use your library of shapes as a reference to build other objects out of, like legos. Eventually you reach a point where you can think and draw fluently in 3d, which is a major asset to artists. I don't want to totally discredit lines, they can be used as a sort of wireframe in building these form studies.
I also suggest you check out what Arne has to say on lighting.
http://www.itchstudios.com/psg/art_tut.htm#light_stuffSo pick a light source and give the rock another go!