Youuuuuu can usually connect to the router by typing 198.162.1.1 in the URL bar of your browser, although it may differ depending on what type you use. Default user name and password is either admin/admin or root/admin on all routers. From there you can navigate to wireless security settings, make them WEP, write down the key (there will be 4, any will do), and then enter that in your DS.
Turn on Pokemon, press Start, select Nintendo WFC Settings. Tap the blue box, Connection 1, Search for access point, select your router when it's found, enter they key you wrote down, and carry on from there.
http://portforward.com/english/routers/port_forwarding/routerindex.htm
Note that if your ISP set up your wireless router/someone else did, they may have changed the default username password (Verizon seems to like to do this a lot). To reset your router to default settings, usually you just hold down the reset button for 15-20 seconds.