Do you want to log every packet or simply alert on signature matches? Capturing every packet on your local network could generate a rather large file.

A command you can use to get started would be: 'snort -dvi eth0' (Substitute eth0 for your NIC information). This is from the Snort FAQ which is located at http://www.snort.org/docs/faq.html

Once you work through the rule logic Snort becomes very easy to work with. Let me know if I can be any further assistance.