If you're going to practice shading, you're going to need a few colors. If you'd rather not go through all the fuss of picking custom colors yourself, you can use this palette. To use it, copy and paste it into MS Paint and use the Eyedropper tool to select the colors you want.

If that's too small, then don't fret - just press Ctrl+W while it's still selected in Paint. When the new window pops up, put 200 in the first two boxes to make it 200% of its original size - or, twice as big. However, you may not need to do this, as you'll probably be working zoomed in most of the time.

Looks shiny, huh? It was actually very simple to do. Take a look at it zoomed in...

Note how the different shades of red are placed.

Now I'm going to crystallize that shiny red fruit...

Whoa! It's crystalline now! (Or it just has another light source, take your pick.) Let's take a look at it zoomed in!

Again, note where the colors are and how they correspond to the smaller image.

Now I'm going to put that cherry in a setting.

The best advice I can offer is to study artwork. Pay attention to how other people shade. If you have to, paste pictures into Paint so you can zoom in and get a better bead on the details.

Here's something for you to study... (yes, you have to blow this one up yourself. No, you may not use it for anything beyond educational purposes. It's my artwork.)