OK. Make sure your MacBook is on and connected to your wireless network. Now, click on Network in System Preferences. Do the following:
-Make sure that 'Airport' is highlighted in the list of network connections on the left-hand side.
-Make sure that your network is selected in the 'Network Name' drop-down menu.
-Make sure 'Ask to join new networks' and 'Show AirPort status in menu bar' are checked.
-Now click on 'Advanced'
-Click on 'AirPort'
-Make sure your network is listed in 'Preferred Networks'. If not, add it by clicking on the little '+' button.
-Check 'Remember any network this computer has joined', and uncheck everything else.
-Click OK.
-Run 'Software Update' to make sure you have the latest AirPort updates (if any).
-Close all programs and reboot.
This should fix the problem. If not, I'll need more info: OS version, router, security, whether it's an open or closed network etc.