Post New Topic and Reply

Is there a way where I can allow the members to reply on the created topic but they can't post a new topic? How?

Admin CP > General > Forums & Categories > Click on 'Permission' button.
Select Advanced mode and there are options that you need.

Write and reply are always together. Can I make them separated? If I allowed them to post in my topic, they can also post their own topic. I don't want them to post their topic there. I just want them to post on my created topic.

If you mean you want to allow them to create topic but not reply in it, then not possible.
You can prevent them from creating topic and allow them to make replies only.

That's what I am asking!
How can I do that. Allow them to post replies but prevent them to make a topic.

Well try this in the permissions of a forum
Reply a message: Check it.
Open a topic: uncheck it

See if that works.

Sanket, thanks for the help. I found out that I should tick the advanced mode. Thanks!