I am a Laotian. where and how to get Hong Kong visa?, in Lao PDR

You can apply for it from the embassy of PRC.
The first thing is to prepare the required materials containing the application form, a recently-taken passport-sized color photo, copy of your passport, transportation arrangement, hotel reservation, proof of your financial support, and your travel plan to HK. And then submit the application to the embassy of PRC to apply for it.