I want registered users to add new posts and comments, but I as admin must moderate all their reactions.
Is there a plugin for user registration and post moderation?
For registration you can use something like this:https://wordpress.org/extend/plugins/front-end-users/screenshots/ or you can do a search here or on any search engine and I'm sure you'll find more than enough plugins. As for the moderation, I have new posts added as drafts and then publish them myself after checking them. Not sure what you want to do with comments since wp allows registered and non registered users to leave comments.
Found this also:http://wordpress.stackexchange.com/questions/7134/front-end-register-form
For new posts, if you (in wp settings) allow your new registered users to be contributors, then they can create posts in draft, but cannot publish them.
The editor role can publish posts.
Comment moderation is set in the settings.
No plugins needed.