No one wants a naked avatar!!! (Well, almost no one.) You have your avatar and now you want to create some clothes for it.
Choose which avatar you want to dress. This article will dress the female avatar because it is more complicated. (It's the curves!)
Fill her torso with white.
Create a triangle. This will be for a bit of skin showing at her neck, so remember that. You can draw one, or create a shape by clicking on the 'Stars and Polygons' icon and then choose star and give it three corners.
Fill it with the same color as her face and put it in the appropriate place. You may need to rotate it a bit.
Change its position. While still selected, hit 'Page Down' and it will go down a layer and stay on top of the dress while under the head.
Choose a color for the clothing and fill that in. This will be the tops and bottoms of her clothes.
Using the bezier tool, connect her waist. You are making the tops of her pants, in effect. Choose a color that you like for her pants.
Do the same thing and add her midriff or whatever you plan to put on her.
To fill an area, once you have added a line with the line tool (making sure there are no gaps), use the paint bucket to fill in the area.