When you go to your local hardware or home improvement store there are several ways to be sure your lock sets are keyed alike.
Sometimes they'll be sold in sets like this. With 2 or more keyed locks or sets of knobs and dead bolts that will all use the same key.
If you need more than are in one set you'll need to look on the packaging somewhere for a number indicating the key type. If you can find another package with the same key number then all of your locks will use the same key.
Here is an example of what it might look like.
|Notice Key # 34025|