create robot model for rviz

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Have you looked at the urdf tutorials ? These explain the creation of a urdf model comprehensively.

Short answer: For the different links (e.g. "parts") of your URDF model you can use either built-in primitives like cylinders, boxes etc. or a mesh in .stl or .dae (COLLADA) format. Meshes in these formats can be created/exported using a multitude of 3D modeling solutions, like AutoCAD, Inventor, 3DS Max, Blender, Sketchup or others. The safest bet is probably to export a plain .stl file at the beginning, as COLLADA meshes sometimes seem to have problems with exporting due to being more complex.

You can also check the http://www.ros.org/wiki/pr2_description package (install it on your machine) and have a direct look at the PR2 urdf files.

It should also be noted that visualization in rviz is only one of multiple uses of a URDF model. Having a URDF model of your robot for example also makes it easy to transform data between different coordinate systems or to simulate the robot in gazebo.