How exactly do i set custom permissions for certain forums? For example, one forum is for news and updates but I don't want members to be able to post new topics only replies, etc.
I cannot figure out how to do this.
If I remeber it correctly, you should check in board settings.
I usually do it with permissions profiles /index.php?action=admin;area=permissions;sa=profiles
I create a profile and then I apply it to the boards I need
Have a look at SMF permissions, their documentation is complete https://wiki.simplemachines.org/smf/SMF2.0:Permissions
There should also be a build in "no replys" permission profile that you can assign as a profile to a specific board.
Agreed. Setting the permission to no reply in board settings is the easiest way.