I just installed Airport Extreme to use with my MacBook Pro. However whenever I close the laptop or restart, I have to reselect my network. With Belkin, the Mac ook always connected. Any ideas?

First contact your ISP. You need to confirm that the issue is not on their end.

Change your router channel. Sometimes this is all you will have to do.

Power cycling the router. Read the router's user manual or contact their tech support for instructions.

System Preferences/Internet & Network/Network

Unlock the padlock

Locations: Automatic

Highlight Airport

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:

Troubleshooting Wi-Fi issues in OS X Lion and Mac OS X v10.6

Configuring 802.1X in Mac OS X Lion and Later

Non-responsive DNS server or invalid DNS configuration can cause long delay before webpages load

Netspot

How to diagnose and resolve Wi-Fi slow-downs

Pv6 troubleshooting

What Affects Wireless Internet?

Solutions for connecting to the Internet, setting up a small network, and troubleshooting

If using one of Apple's Airport routers, read its user manual or post in its forum area.

If using a 3rd party router, read its user manual, contact their tech support department/website or post in its forum area.