@XiaohuiChen
There are a number of ways instructors provide students access to discussions. The Discussions tab may or may not be present in the course navigation. If the Discussions tab is not there, the instructor may provide the link to the discussion in a module.
If you aren't able to access a discussion topic from one of these places, you'll need to send a message to the instructor to ask for guidance. It's possible that the instructor has set availability dates on the discussion making it unavailable for participation until a specified date.
General instructions for participating in a discussion are in How do I reply to a discussion as a student?
