You add level adjustment and hit dice to get effective character level. A centaur from the monster manual has 4 hit dice and +2 level adjustment. Add them together for an ECL of 6. This means that the centaur as presented in the monster manual is roughly as powerful as 6th level character of a core race from the player's handbook.
Say you want to play a centaur character, beginning with 1 level of ranger, and then increase your ranger levels normally as you gain experience during play. Add 4 hit dice for being a centaur, 1 hit die for your 1 ranger level, then the +2 level adjustment, for an ECL of 7. Look on the 7th row of the Xp table, the row for a 7th level character. You have 21,000 experience points. To advance to your next ranger level, you need 28,000 Xp, same as an 8th level character of a race from the PHB. For 3rd ranger level, 36,000 Xp and so on.