If you’re just getting started you can try some of 3D modeling software which can be downloaded for free.
Google SketchUp – This Google SketchUp is fun and free, and is known for being easy to use. To build models in SketchUp, you draw edges and faces using a few simple tools that you can learn in a short time. With with Push/Pull tool you can extrude any flat surface into a 3D form. Furthermore, it works together with Google Earth, that you can import a scaled aerial photograph directly from Google Earth, or use SketchUp to build models which can be seen in Google Earth.
3Dtin – The simplest 3D software. You can draw directly from your browser.
Blender – Blender is the free open source 3D content creation suite, available for all major operating systems under the GNU General Public License. Blender was developed as an in-house application by the Dutch animation studio NeoGeo and Not a Number Technologies (NaN). It is a powerful program contains features that are characteristic of high-end 3D software.
OpenSCAD – OpenSCAD is a software for creating solid 3D CAD objects. It is free software and available for Linux/UNIX, MS Windows and Mac OS X. it does not focus on the artistic aspects of 3D modelling but instead on the CAD aspects.
Tinkercad – Tinkercad is a new and faster way of creating designs for your 3D printer. With only three basic tools you can create a wide range of useful things. Once your project is ready simply download the STL file and start your 3D print.
You can also draw 3D models using any more complex 3D modeling software like 3D Studio max, AutoCAD 3D, Maya, Rhinoceros, Solidworks etc.