You can download Wireshark for Windows or Mac OS X from its official website. If you’re using Linux or another UNIX-like system, you’ll probably find Wireshark in its package repositories. For example, if you’re using Ubuntu, you’ll find Wireshark in the Ubuntu Software Center.
Just a quick warning: Many organisations don’t allow Wireshark and similar tools on their networks. Don’t use this tool at work unless you have permission.