I am a bangladesh citizen wishing to reach San Francisco via Hongkong. Do I need transit visa? If ye

Majid, yes, you need it and you should apply for one in advance.
You need to prepare those materials including copy of your passport, the application form, a passport-style photo, proof of your financial support, transportation arrangement, and proof of the proposed visit in the next destination after transiting in HK. And then submit the application to the embassy of PRC to apply for it.