I know how difficult it can be to shade pixels in Microsoft Paint- it took me almost a year to find a realistic style of shading, so I made this tutorial to help you to shade effectively. Please do not copy the pixel featured in these examples.

Step 1:
We'll start off with the shadow. Open up your pixel in Paint and zoom in to 8x.



Using the 'Pick Color' tool, click on the main colour of your pixel. Now go to 'Colors', click 'Edit Colors', then click 'Define Custom Colors'. Drag the arrow on the right down to make your colour a few shades darker.



Step 2:
Now imagine that your pixel is an object; where would the light not hit it? Which areas of the pixel would be in the shade? When you've found the areas that need to be shaded, begin colouring them with your darker shade.



Step 3:
The trick to shading pixels realistically is to blend your colours well, in my opinion. To blend the colours, make your darker shade a tiny bit lighter, then go around your shading with this colour (see the example below). Keep blending until you get an effect that suits you.



Step 4:
Now you just need to highlight certain areas of your pixel. Imagine where the light would hit your pixel- these are the areas that you need to highlight. Again, using the 'Pick Color' tool, click on the main colour of your pixel and make this colour a few shades lighter. Then colour in the areas that you need to highlight. Don't be afraid to use a shade that is much lighter than the main colour of your pixel because this can give a really nice 'shiny' effect, but remember to blend your colours to make the shading look natural. Continue to shade and highlight the appropriate areas of your pixel until you're satisfied with the effect.



Step 5:
Then just zoom out and you should have a nicely shaded pixel!