Here are links to three of the most popular places to register for baby gifts:
Babies R us/ Toys R us
You can also register at other web based sites if you have a preference for specific types of baby items, or at local baby stores if some of the people who will be visiting your registry are local.
Different places often have completion offers or other deals for those who create a registry at their store/site, so it is good to see what each place is offering at the time you are planning on creating a registry to see which place has the best deals for you (for example, some may offer you a discount on the items that are left on your registry one week before your due date, so you can purchase it yourself. I know Target has had an offer where they give you a little gift kit for signing up for their registry.)