If you goto http://www.blender.org/ you can download a free 3rd party software which will allow you to create a custom avatar modelling and texturing.
Here is a great place to start to learn about mesh creation http://blog.machinimatrix.org/3d-creation/blender-meshes-trail/meshes-overview/
There is a lot to learn when it comes to 3d modelling, if you have no experience in this it will take a lot of time. Please be advised that if you do create a custom avatar, all clothing and attachments, especially texturing will have to be done using the UV unwrapping layout you achieve... basically, it will also have to be done by yourself, a friend or comissioned to someone else.
It is not exactly a simple process to create an amazing custom avatar, in essence you are talking about several days or weeks of work for a professional to create one custom avatar with an outfit. It is a full 3d model textured. If it is not humanoid, then an outside animator such as poser might be required and this takes even more expertise and time.
If you decide to go down this path it may be a lot of fun and a great experience, but it wont exactly be painless, there is a lot of dedication that comes with excellent content creation!
-Kiwe