To play in Oberin, you need to create a character. To create a character, you need to use the Oberin Character Creator. This is a separate program that should be in the same "Oberin Client" folder as Oberin itself. You should have read the system requirements and character name rules before you downloaded Oberin. If you are creating a character, then you want to read How do I get started? on the official Oberin site. You might also want the Character Creation FAQ.
Another use of the Character Creator is to check the max skills for each class and to experiment with stats. When using the Character Creator this way, you should not fill in any fields except for "Character Class" and skills, and you should not click the Create button.
From time to time, the server admins will disable the Character Creator. Then when you open it, you will see a message that it is unavailable and then it will close itself without allowing you to do anything. While you wait for it to come back into service, you can read about the six classes here at OberinWiki: Fighter, Ranger, Rogue, Wizard, Cleric, Druid.