I have just joined bitcoin talk for a while. Although I just go to see information and do bounty, but I still do not understand how to get merit and how to get it. please explain to help me understand this problem. thank you all.
You will not get merit if you just "do bounty" and you don't really need it.
Beyond that, if you participate and contribute in discussions you will likely earn merit but if you try to earn merit you will likely not. Go to e.g. Bitcoin Discussion and read a few larger threads - you'll see what kinds of posts get or don't get merited.