After you've designed your character, you need to turn it into a 3D model. This model will certainly need to look good, but it must also meet some technical requirements. Most models need to be deformed during animation, so your model must be constructed in a way that allows it to deform naturally and easily. Building your character properly before you start animating will save you headaches in the long run.
Modeling is an art unto itself, and in many ways, it's like digital sculpting. Becoming a good sculptor requires a good eyeand a good grasp of volume and form. And, as with all arts that center on the human form, it's a big help if you have a good understanding of anatomy. Taking time to learn sculpting in a physical medium, such as clay, can also improve your digital sculpting skills.