One thing John failed to mention: CHANGE THE ADMIN PASSWORD!!! Manufacturers have a default password so the buyer will have some way of getting into the thing, but far too many people just leave it. Don't be one of those people! As John said, a dedicated hacker will eventually be able to read your traffic, but he won't be able to make any changes to your network if he doesn't have the admin password.
If you have modern hardware, and if the D-link router supports it, use Wi-fi Protected Access encryption. This is lightyears ahead of WEP (which isn't too difficult to break), and once you set it up it stays secure. Sure, it can be a little bit of a hassle to get going with WPA, but it's worth it.