Best place to learn custom Avatar creation?

Any advice or help is much appreciated.

What do you mean? Making avatar attachments from prims? Customizing your shape and appearance? Making custom skin and clothes? Creating custom animations? All four are skills you use either alone or in tandem to customize your avatar. They are all different skills.

Reading this forum is a good place to start for the prim work, try the texture forum for skin and clothes, and the animation forum for animations. Try running a search in these forums for avatar shape, I'm sure there are good threads for that too.

Best of luck...