Babies can ask for things by asking for things that they want. Teach your baby the sign for the word want in baby sign language with the following description and video.
Form the hands into claws. Hold the hands in front of the chest with the palms facing up. Pull the hands in towards the body while closing the claws into fists.
Beyer, Monica. 2006. Baby talk: A guide to using basic sign language to communicate with your baby. New York: Jeremy P. Tarcher/Penguin.