I just wonder that, if I creat a good topic but only Jr.member or lower join how can they give me Merit? (they don't have it).
You can create a great and helpful topic in your local board. Based on how you construct your post, sticking on the local board is the best thing I can say to you.
A thread is viewed by everyone and not only based on your rank, you can't filter a thread just for Jr. Rank Members. If you create a great topic, don't expect that someone will be giving you a merit since not all of the members of these forum are interested and find your post constructive and worthy of merit.
And if I Merit someone, the merit of me is descrease?
Giving your sMerit does not "decrease" your Merit.