To design 3D models I decided to use the Blender open source 3D software, mostly because its free and there are many online tutorials available.
Setting up Blender was a bit of a challenge since my computer is a Mac and Blender’s default settings are designed for a 3-button mouse. But there are online instructions for how to make things work on OSX with a Apple touch mouse.
I purchased a book called “Blender 3D Basics” as a beginners guide for Blender. I worked through most of the book and learned how to model a sloop sail boat. The book has some chapters on animation, but I’m not interested in that yet so I skipped those. I would recommend the book as it describes each step in detail and there are many useful techniques that you will learn to do 3D modeling.
There are also many video tutorials on YouTube. Unfortunately, many of them are for older versions of Blender. Blender’s user interface has changed dramatically over time. So the instructions are difficult to follow since they don’t map to the newer user interfaces. The best is to be very specific in your searches on YouTube to look for a specific main version of Blender like “2.6”. YouTube is good for finding out how to do a specific task or modeling technique.
If you are interested in video tutorials that will cover the entire design process for a particular kind of model, then Blender Cookie has a paid tutorial service. I’ve done several of their tutorials and I can definitely recommend them. The only issue I have as a Blender beginner is that the instructors do things very quickly in the videos. But I just pause the videos a lot to catch up, so getting through the tutorials will take much longer than the length of the videos.