Here are a few options for preparing a model file for CNC work:
Luban Case Library
Use provided designs in the Luban Case Library for beginners.
Design:
You can design a 3D model using modeling software such as Fusion 360 and AutoCAD. You can also design a 2D image for cutting only.
Download:
You can find free CAD files, 3D image & Model from the website like grabcad.com/library.