Visas to China, Hong Kong and Macau

I am Cambodian, currently in France. I am planning to do my internshing in Guangzhou. I know that Visa is required for me to stay in mainland. But during my internship, I would also like to visit Hong Kong and Macau. So how can I apply them?, which criterias do they require to get them? Where can I apply for them in Mainland ? and how much do they cost? Moreover, If I have only a single entry for Mainland, So how can I do for getting back? Where can I need to apply a new one? Sould I apply it before leaving to Hong Kong or Macau? Thank you for your help in advance.