Is this leaf moving or static? Because this would be pretty advanced stuff. You should probably make it static at first at least.
I started out just trying to make a few basic games, like 2D platformer, top down shooter, Pong, Space Invaders, a car driving game and such.
You can take the 3D leaf object and give it a mesh collider, and it should make it possible for the catepillar to move around on it. Animating it is probably best done in a 3D program like Blender, but if it already has an animation then it should be pretty much drag and drop.
Here is a small video about important a 3D object into unity with an animation: