How to shade:

I will try to explain how I do my shading. This is my first tutorial, so *cross fingers* :)
I use Adobe Photoshop.

I will work on this base, and make a dress.
The 3 purple dots are the colors I will use.

STEP 1:

Draw the outline of your dress, using the pencil tool.
 Make sure there are no "holes", so all pixels meet.
I like to put it on a seperate layer.

STEP 2:

Using the magic wand tool, mark off the area inside the outline.
Do not release the wand until finished.

Step 3:

Using the paint bucket tool, fill the marked area
Use the lightest of the colors..
I like to put it on a seperate layer.

STEP 4:

Use the Paintbrush tool and the middl color to draw the midtones.
Use quite large brushes at first, then some smaller ones.
The smallest I use is brush 5.
I like to put it on a seperate layer.

STEP 5:

Using the darkest color, make the darkshades.
Start by following the outlines.
Use brushes around size 5 and 9.
Work it until you are happy with it.
If you have used seperate layers, you can use the paintbrush eraser and such to perfect it.

Ta daa! All done! :)