I want to go to China for 1 month to travel, do I need separate visa for Hong Kong, Tibet?

You do not need a visa to Hong Kong, but you are required to apply for a visa to China and if you want to visit Tibet, you need to apply for a Tibet Travel Permit from the Tourism Administration of the Tibet Autonomous Region (Tel: 0086-891-6834313; Fax: 0086-891-6834632). Besides, individual tourists are not allowed to enter Tibet, you need to join a tour.
The validity of China visa is 3 months, so you can apply it 1 or 2 months earlier before your departure. You can not do it online, you need to submit your application to Chinese embassy or consulate where you reside, and you may required an interview if necessary.