Beginners with low powered Computers can start with these tools.
You may try Sketchup make 2017. Or try the new SketchUp for web. It’s the easiest 3d software I ever used. Personally use SketchUp 8. You’ll find all the old Sketchup version in Firehose (third-party, be careful).
Autodesk Tinker CAD, totally free and runs on the web. Very friendly UI and easy to learn. Popular among School Kids. But saw some pretty complex 3d model made with this tool.
Wings3d is a good Polygon Modeler and open source.
Blender3d is the most successful Open Source project in the Computer Graphics realm. Very small size and you don’t even need to install it. Just download the zip version, extract, and use. For low specification computers try the old version from THIS link.
If you are serious about 3d modeling and rendering tasks my recommendation is to use a higher spec pc. And use the Industry-Standard toolset according to your career path.
Thanks and Best Wishes.