When you start with 3D modelling, you can start with low-poly style since it has the least amount of detail required thus get you efficient.
The skills to create 3D illustrations are mostly the same with drawing: observation, spatial reasoning, compositions, and a sense of detail.
Once you're familiar with the low-poly style, you can then add more details to move into a realistic style or exaggerate them to create a stylized version of the model.