Use the basic statistics for a creature of the familiar's kind, but with the following changes.
Hit Dice: For the purpose of effects related to number of Hit Dice, use the master's character level or the familiar's normal HD total, whichever is higher.
Hit Points: The familiar has half the master's total hit points (not including temporary hit points), rounded down, regardless of its actual Hit Dice.
Attacks: Use the master's base attack bonus, as calculated from all his classes. Use the familiar's Dexterity or Strength modifier, whichever is greater, to calculate the familiar's melee attack bonus with natural weapons. Damage equals that of a normal creature of the familiar's kind.
Saving Throws: For each saving throw, use either the familiar's base save bonus (Fortitude +2, Reflex +2, Will +0) or the master's (as calculated from all his classes), whichever is better. The familiar uses its own ability modifiers to saves, and it doesn't share any of the other bonuses that the master might have on saves.
Skills: For each skill in which either the master or the familiar has ranks, use either the normal skill ranks for an animal of that type or the master's skill ranks, whichever is better. In either case, the familiar uses its own ability modifiers. Regardless of a familiar's total skill modifiers, some skills may remain beyond the familiar's ability to use. Familiars treat Acrobatics, Climb, Fly, Perception, Stealth, and Swim as class skills.