Depending on how the administrator setup his/her forum, you may have to register and login before doing this.
After that, you have to have posting permission to be able to do this on any given forum. If you do not have posting permission, then you will not see this option available to you in the viewtopic page.
If you do have permission, then in viewboard or viewtopic, you will be able to either Post a new message or Reply to an existing message by clicking on the specified posting button.