I want to jion two plaste board that are hung on wall.
Taping and finishing drywall seams is a art! You can do it yourself if you take your time-it's not a job that gets done in one day! The corners can be time consuming and frustrating! Here's some information for you to give you an idea of how to do the job!
Here is a video on how to tape and plaster drywall. When you pull up this video, there are others by the same guy to explain all the questions you have.
I'm not sure if you are talking about drywall or another product.