Change your router channel. Sometimes this is all you will have to do.
System Preferences/Internet & Network/Network
Unlock the padlock
Click the Assist Me button
In the popup window click the Diagnostic button.
System Preferences/Network- Unlock padlock. Highlight Airport. Network Name-select your name. Click on the Advanced button. Airport/Preferred Networks-delete all that is not your network.
Place a check mark next to "Remember networks this computer has joined." Click the OK button and lock the padlock. Restart your computer.
http://support.apple.com/kb/TS1920 Mac OS: How to release and renew a DHCP lease
No internet connection (wireless)
Check to see if an extra entry is present in the DNS Tab for your wireless connection (System Preferences/Network/Airport/Advanced/DNS).
Delete all extra entries that you find.
Place a check mark next to "Remember networks this computer has joined."
Other resources to check into: