Most popular search tools:
There are lots of websites that let users post their own 3D models for anyone to download. You can search these websites to find a design you like and download the file for free. The file format you download will be .stl.
Here are a few examples sites:
Below are few examples of free CAD software programs that are suitable for beginners. These programs also come with helpful tutorials to get you started.
See here for a more detailed breakdown of available CAD software for beginners, intermediates, and professionals.