Education.com has articles that you ought to read.  Type "talking race" into the search box at the top of any page and pick an article or two.  I put a link below to one article that directly deals with the topic.
 
This can be more difficult if you live in place lacking racial and ethnic diversity.  I was fortunate in that regard having lived part of my childhood in a suburb of San Francisco in the 60s, and having ancestors active in the actions around the American Civil War.

try maybe showing him or her different pictures of diffrent races, and ask them if they can tell what makes them different. then when he or she says ohh he's black so he's from africa or she's from japan, just make sure she gets the details right and show her what the un-visible details are, like the foods they eat and homes they live in. Kids love eskimos, id start their.

National Geographic, Discovery and other similar channels have great programs that expose us to different cultures, their traditions and rituals.
I think this is a great place to start. There are many DVD's and TV shows that will keep your child fascinated or at least interested.
 
If you have the financial means, travel to other countries would also be an amazing experience to your child although, This might be better when they are a little bit older so they retain the experience.

get some books about races. i have seen alot .