"Make sure your wirless adapter is set to dynamically obtain IP address"

In System Preferences - > Network, select your Wi-Fi network adapter and click the Advanced button.

In the resulting dialog, click the TCP/IP tab.

Here, make sure the 'Configure IPv4 address is set to 'Using DHCP'.

This is the default for most networks, so the chances are you don't have to do anything, but that's where it's set.