Unlike skins, avatar shapes can be customized by the average user in Second Life. The shape and skin combined give the avatar its appearance.
Linden Lab® Avatar Shapes
Linden Lab® provides quite a few free avatars in the Library part of your Inventory. You can mix and match various pieces of these avatars to get started with creating your own custom look. The pictures in this folder show the various full-perm shapes in the Library with the same skin so you can see the differences in the shapes. The model is wearing the accompanying Eyebrowshaper or system hair, too. The boxer shorts on the shapes are separate.
To access the Linden Lab® outfits, open Inventory. You’ll see two main folders, Inventory and Library. The Library folder contains all of the things provided by Linden Lab® for you to use in Second Life®. Open the Clothing folder in the Library to see the folders with outfits. When you drag an outfit folder onto your avatar, you will be wearing everything in the folder, and a copy of the folder and its contents will appear in your Inventory, in your own Clothing folder. Once you have done that for several (or all) of the Library outfits, you can mix and match whatever pieces you want.
Modifying Avatar Shapes
You can modify any of these shapes by wearing the shape, right-clicking on it in Inventory and clicking Edit, then using the sliders to adjust each body area. Make sure you save the altered shape as a different name. The skin that you choose will alter somewhat how the shape looks. You can also create a completely new shape by clicking on the + sign in Inventory, then choosing New Shape.
Second Life QuickTips – Customizing Your Appearance – YouTube
Controlling your avatar’s appearance – Second Life
The Basics | Second Life Avatar Personalization