Level Adjustment and Effective Character Level: To determine the effective character level (ECL) of a monster character, add its level adjustment to its racial Hit Dice and character class levels.
Use ECL instead of character level to determine how many experience points a monster character needs to reach its next level. Also use ECL to determine starting wealth for a monster character.
Monster characters treat skills mentioned in their monster entry as class skills.
If a monster has 1 Hit Die or less, or if it is a template creature, it must start the game with one or more class levels, like a regular character. If a monster has 2 or more Hit Dice, it can start with no class levels (though it can gain them later).
Even if the creature is of a kind that normally advances by Hit Dice rather than class levels a PC can gain class levels rather than Hit Dice.