Create an artboard in Illustrator of the size of your video, e. g. 1920×1080 px.
After creating your graphic, consider which pieces of it will need to be controlled in After Effects. You may find that you want control of every single element, or it might make more sense to keep certain elements together. Once you've decided what you want to animate, you'll need to separate them out into different layers.
You'll need to separate each part (sub-layer) into its own layer.
Please make sure that your layers are in the order you designed them, so the correct elements are on top.
Appropriate naming the layers is a great convenience for the animator!