I plan to visit China & HK as well, what to do next .

You only need to apply for a Chinese visa. If you plan to visit HK and return to mainland China, then you need a double entry. Round-trip ticket and hotel bookings are required for application. Basic requirements include your passport, application form with photo. Travel insurance is also helpful if any.

Thank you !
Apply online ? in person ? via third party ? if via third party .... then any contact details ?

You can apply through a travel agency, appointed by the Chinese embassy. Please check official website of Chinese embassy for the list if the agenies.