See that cool help button at the very top of your screen? Anyway right out of there.
Choosing preferred AirPort networks
By default, AirPort attempts to join the wireless network your computer was most recently connected to.
If you are in an environment with multiple wireless networks, you can set up your computer to join a preferred network when you start your computer, wake it from sleep, or turn AirPort on.
AirPort keeps track of the wireless networks your computer has connected to, and lists them in the Network pane of System Preferences.
To choose a preferred AirPort network using Mac OS X 10.4:
Open Network preferences, choose AirPort from the Show pop-up menu, and click AirPort.
Choose "Preferred networks" from the "By default, join" pop-up menu, and select a network from the list.
You can add wireless networks to the list by clicking the Add (+) button, and clicking on the Network Name pop-up menu. Choose a network and enter a password for the network if necessary. Use the Delete (-) button to remove a network from the list.
To give a network the highest priority, drag the name of the network to the top of the list. This is the network AirPort will attempt to join first.
If you are using Mac OS X 10.3, choose "A specific network" from the "By default, join" pop-up menu, and either type the name of the network in the Network field, or click the Network pop-up menu to display the available networks, choose one and then enter your password for the network.
You can set additional options to control the way your computer joins available networks. Click Options and choose an option from the "If no preferred networks are found" pop-up menu.
If you choose "Ask before joining an open network," you will be prompted when an open network becomes available. If you choose "Automatically join an open network," you will not be promoted, and will automatically join an open network when it becomes available. If you choose "Keep looking for recent networks," AirPort will continue looking for networks you have already joined, and automatically join the network when one is found.
Never do anything you don't want to explain to the paramedics.