It will still be a while before The Sandbox is ready for players, but you can design your own avatar right now! A guideline and several sample models have been published on Medium.
The sample models are already broken up into correctly sized objects, so you can easily make your own creation. By the end of the week I should be done editing down my three hour live-stream of animation rigging in VoxEdit, and you can learn how to rig and animate your own character.
I imaging that in The Sandbox, there will already be a template for characters so you won't have to animate your own avatar, but it's fun to learn how to do your own animations. It will be an important skill once the game's marketplace is open for player creations.