Characters in the Worlds of Adventure campaign are similar to characters in D&D 3rd edition or in Pathfinder. The pages linked below contain all the information necessary for creating a new character or adding levels to an existing character.
Character Creation: All about creating characters, either at 1st level or at a higher level.
Note: Each player's first Worlds of Adventure character must be a 1st-level character. Subsequent characters may be of higher level; see "Creating a Character Above 1st Level", on the Character Creation page, for details.
Backgrounds & Traits: Options for character backgrounds and character traits. (Each character gets one traits, and only the traits listed there are available.)
Character Advancement: All about gaining levels. Also describes the multiclassing and cross-classing rules used in the Worlds of Adventure campaign.
What Level?? Explains how character level works in the Worlds of Adventure campaign, which is a little different than in Pathfinder or other similar games.
Simple Character Creation: A simplified version of the character creation process, for quick reference.