I don’t think Trust Levels would be your solution here. You can use the built-in groups for this purpose.
Discourse can give different groups of users permissions on a per-category basis.
If you want don’t want users to create new topics in those categories then make sure the ‘everyone’ group or the equivalent group for your users has ‘Reply/See’ and not ‘Create/Reply/See’. This is set up in the Security tab of the category settings.
If you’re an administrator you don’t need to give yourself the Create permission, it’s implicit, but if you aren’t an administrator then do make sure whatever group applies to your user has ‘Create/Reply/See’ granted to it in the same category.