A conversion script (Arnold > Utilities > Convert Shaders) that converts legacy Standard shaders to Standard Surface shaders is available in the Arnold Utility menu of MtoA. This simple script also converts unsupported materials (Phong, Blinn, mia_material, etc..).
Due to a large number of controls, the Standard Surface shader is split up into several groups. The individual settings for each group are described in more detail in the pages below.
The MtoA material library can be found here.
Further information about physically based rendering in Arnold can be found here.
When rendering diffuse surfaces, it is very important that the normals of the geometry face in the right direction. In the example below, you can see the difference between normals that are facing inwards in the wrong direction (left side) versus those that are facing correctly in the outwards direction (right side).