In Mac OS X, we have a simple way to enable and disable the wifi, as well as to connect to a wifi network. If you look in the menu bar on your Mac, that's the bar that runs along the top of the screen with the various menus which relate to the currently active application, you will see the Airport symbol.
Where you see the padlock, you can tell that the wireless network has a security key (i.e. a password) that you must enter in order to be able to connect to this network. In older versions of Mac OS X, the padlock was not shown in this menu. If you select a wireless network and it asks you to enter the password or security key, then you know that it is locked.
Generally, you will want to connect to your own wifi network at your home or workplace. To do this, simply click the Airport icon in the menubar and look for the name of the network that you wish to join.
Click the name of the network and wait. If it is secured with a password, which most networks nowadays are, then you will be presented with a window which asks for the password for that network. Enter the password and click the Join button.
You have a couple of other options on this window. Firstly, you can click the Show Password button which will show you what you are typing, instead of the default which is to show bullets, so that someone looking over your shoulder can't just copy your password. Secondly, the Remember This Network option (which is selected by default) will remember the password you enter, so that the next time your Mac will be able to automatically connect to this network without asking you for the password. Usually, you will want to do this. If not, just uncheck that box.
Once you have clicked the Join button, you will notice that the bars of the airport icon start to move, as it searches for the network and attempts to connect. When a successful connection is made, the icon will change to the icon shown above (Airport On), with some or all of the bars in black. The number of bars that display in black on the icon indicate how strong the signal is. If you have all 4 bars in black, the signal is excellent. This will change from time to time as the signal strength increases or decreases.
You can disable wifi on your Mac completely by simply clicking on the Airport icon in the menu bar and choosing the Turn Wifi Off option. This will disable your airport card until you choose to re-enable it. To switch it back on, just do the same thing - but this time the option will say Turn Wifi On.
There is one more very important thing to know, which many people have problems with and don't know how to solve. What happens if you have connected to a network and it all works fine and the password is saved for future connections to that network - and then someone changes the password for the network? One indication that this has happened can be that the Airport icon shows an exclamation mark (!). This basically means that you are connected to the network, but you have no IP address. Without an IP address, you can't really do anything on the network. There are other reasons why this can occur, but most commonly it's because your Mac has the wrong password saved. This can be because you entered it incorrectly (yes, sometimes it will accept a bad password, but you won't be able to go online), or because someone has actually changed the password for the network you are trying to access.
In order to be able to re-enter the password, you need to remove the saved password that the Mac has on file for that network. This is relatively simple to do. First, go to the Airport icon in the menu bar and select the option Open Network Preferences. This will take you to the Network pane of the System Preferences window as shown below:
This window list the networks that are saved on your Mac. To remove the password for any network, simply click the network name in the list and click the "-" symbol below. This will remove the saved password for that network. Click OK and then reconnect to the network as you would to any network for the first time. You should be asked for the password again and provided you enter the correct password, you should be connected!