This article details how to connect to a wireless network on a computer running Mac OS X.
Firstly, click the Wi-Fi icon in the top right corner of the screen and make sure the first line says "Wi-Fi: On". If not, click Turn Wi-Fi On.
Now select the network you wish to join from the list of available networks.
You will now be required to enter the security key if the network is protected. This is usually found on the router itself.
You should now be connected to your chosen wireless network. Wireless signal can be viewed by the amount of black segments in the Wi-Fi icon.