All of the avatars provided by Linden Lab in the Library come with clothing and accessories that can be mixed and matched any way you want to. There’s an amazing variety of clothing, shoes, and accessories (even a mesh Bento horse!) that’s all free.
Using the Library
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.
Mesh vs. Classic Avatars and Mesh vs. Classic Clothing
To me, mesh is like molded plastic. Whole avatars can be mesh, different body parts can be mesh, hair can be mesh, clothing can be mesh, and shoes and jewelry are often mesh. Mesh items need to be designed outside of Second Life and imported in. Mesh clothing usually comes in standard sizes. If the clothing does not cover your whole avatar and bits stick out (as is usually the case) you need to wear an alpha layer. The alpha layer makes the body part invisible so the only thing that’s seen is the mesh clothing.
Classic
Two notes:
1) Feel free to try the female outfits to see what looks interesting, too. For instance, Rhiannon includes a different Bento horse, and there are some shoulder pets and followers too.
2) The older boots may need an alpha layer to look right in the Firestorm viewer, which does not support the invisiprim technique that the older boots are made with.
Ruth is included here just for the SL cultural reference:

Mesh Avatars
There are quite a few creators making mesh avatars and parts of them (heads, feet, hands, etc.) Like mesh clothing, they work using an alpha layer to hide the classic avatar by making it transparent, then you wear the mesh avatar (or part) over that. Besides my personal philosophical objections to standardized avatar shapes, mesh avatars can be tricky to clothe. The following are the Linden Lab mesh avatars. Mesh avatars cannot be separated into shapes and skins; it’s kind of like sticking a doll’s body on top of the avatar body.