There are plenty of software out there, both commercial and open-source ones.
If you are new to 3D modeling, we suggest starting from one of the below software, which have friendly interface and are capable to build simple models quickly.
1. Voxel builder http://voxelbuilder.com/
2. Autodesk 123D Design http://www.123dapp.com/
3. TinkerCAD https://www.tinkercad.com/
And trust me, after you are more familiar with the tools, you will definitely want more tools to play with. At the moment, jump to more professional software.
1. Fusion 360, SolidWorks, Autodesk Inventor and Creo Parametric (Pro/E) are often used by
engineers for product design and can provide accurate modeling.
2. Zbrush, Blender, SculptGL are popular among artists, and are capable of producing
more fluid 3D design.
3. OpenSCAD is a code-based CAD software, which offers a different interacting way
from above software.
There are more software that we don't have the chance to introduce in this article. Try them and look for the one that fits your requirement most!