To sign first, form the number 1 sign with your dominant hand held up to the side of your face, then curl it down and inward (as if forming the letter 'J'). Alternatively, you can hold up the number 1 sign with your dominant hand, while holding up your fist with thumb up on your non-dominant hand. Then bring the dominant '1' hand down to flick the non-dominant thumb.
Teach your toddler the first sign when they are ready to understand ordinal numbers. If you have more than one child, you can indicate who among them is the first by virtue of birth order. Then repeat the concept with your child's first shoes, first tooth, first teddy bear, and so on.
Teach your toddler this sign in conjunction with other ordinal flash cards, once they have grasped the basic concept of the cardinal numbers.