You need to be more specific but for now try this..
1) shutdown your computer.
2) unplug both your router and cable modem(assuming)
3) plug your modem back in and wait for it to reset itself, then plug your router in and wait for that to reset itself.
4) turn computer back on and check to see if you have a connection in your network preference
I have AT&T DSL and my WiFi works from across the street, but I had to load the software disk to my BlackBook 2.4 Penryn in order to get access to my WiFi. It's a 2WIRE AT&T High Speed Internet Version 6.1ACS2W CD. Once the software was loaded to my BlackBook, I was able to set-up my WiFi account by entering my account number and my WEP password. Who's your carrier for DSL or Cable?
You might need to go to "open network preferences" and click on "network name:" your network should be on that list. Sometimes the Airport will pick up and join 5-10 different networks, but it's important that you select your known network to join, then click "advanced" and you can see "preferred networks" and yours should be listed, and I always click the box "remember any network this computer has joined." You should be good to go.