Do I need to get chinese visa to travel in shenzhen and beijing china?

I want to tour in China particularly in SZ and Beijing next year.I want to get Chinese visa in the Phils, one of the requirements is the Invitation Letter for Tourist issued by Authorized Travel Agency in China,may i know what are those authorized agencies? what if i do not have that invitation letter do they still issue chinese visa? How many hours from SZ to Beijing? is it by train or bus? Thank you so much