I'd like to have two levels of player permissions on my Bukkit server:
- Admin: can run all commands
- Normal: can join, chat and walk around; nothing else
How can I set this up and warn players who try to break blocks?
You'll need to use a permissions plugin. Effectively you'll create two groups - one which is 'admin' and one which is 'user'. You can look here: https://forums.bukkit.org/threads/the-ultimate-guide-to-permissions.80291/
However, it'd be easier to just set the default gamemode to Adventure and then manually promote people by changing their gamemode to Creative. You can see that here: http://minecraft.wikia.com/wiki/Category:Gamemodes