Many of an eidolon's attribute modifiers are set, but when you choose your eidolon, you decide their key attribute. Choose Strength or Dexterity as your eidolon's key attribute. They gain a +4 attribute modifier for that attribute, and a +2 attribute modifier for the one you didn't choose. Your eidolon's other attribute modifiers are listed in their entry under Eidolon Attributes.

This also sets your eidolon's defenses, as represented by an item bonus to AC and their Dexterity cap. If Strength is their key attribute, your eidolon has a +2 item bonus to AC with a +3 Dexterity cap. If Dexterity is their key attribute, your eidolon has +1 item bonus to AC with a +4 Dexterity cap. The appearance of the eidolon's protection can be anything you choose that fits the eidolon: armor, scales, tough hide, a mystical aura of deflection, and so on.

As noted in your attribute boosts class feature, your eidolon gets boosts to their attribute modifiers starting at 5th level at the same time you do. They also increase one modifier by 1 when they gain their transcendence ability.