How you handle introducing the new baby to your children will depend on their age. A toddler might be totally indifferent and unfazed, being more interested in your new baby's toys than the fact that she has a new brother or sister. An older child might be shocked and jealous by the arrival of a new baby, who appears to steal her limelight, and usurp his mommy and daddy's attention.
It's a good idea to prepare your little one several weeks in advance, explaining that the new baby will need a lot of time and attention, will need to be fed and changed regularly, and probably not be much fun for a few months. Focus on how your child can be a great helper, and show her what the new baby will need when she is born. Take some books out of the library that explain how families change when a new baby is born, and ask your child to talk about how she's feeling, and how she thinks things will be different when the baby arrives.
Encourage your child to choose a gift to give to the baby, and find something your child really wants as a gift from her new sibling. Ask grandparents or close friends to arrange a few treats or outings for your child both now and after the birth, so that she's occupied, and getting extra attention.