Some babies take to a sippy cup immediately, and some take a while to get used to the idea. (Some may never use one.) Here are some tips for introducing the sippy cup:
Start off with one that has a soft, pliable spout because this feels more familiar to your baby than a hard plastic spout.
Show your baby how to raise the cup to his mouth and tip it up to drink. Show him that the spout is like a nipple by touching the tip of the spout to the roof of his mouth to stimulate the sucking reflex.
Give it some time. Until your baby masters the technique, you may want to put only water in the cup to avoid too many messes. And don't worry if your baby doesn't take to the sippy cup right away. Just wait a few weeks and try again.
Shop around. There are all kinds of sippy cups, with all kinds of spouts. Sippy cups aren't too expensive, so it's worth letting your baby test-drive several if one isn't working. Try different models until you find one that suits your baby.