i am bangladeshi want to visit tibet and some other places in China. how can i get visa?

You can come to the Chinese embassy in your country to apply for a L type for your trip to PRC. If you plan to visit Tibet, you should join a group tour arranged by qualified Chinese tour agency because individual travelers are not allowed to enter Tibet. By the way, the agency will help you get the Tibet Travel Permit. Without it, you can't enter Tibet too.