There is no formal format for a public address of this nature. It's best to tailor it to the specific function. Generally you want to welcome any special guests, everyone else and be sure to mention the event/why everyone has gathered. You should talk in a bit more detail about the event or it's planning etc before concluding your address. Any relevant anecdotes or quotes can be included as well. If you want a general structure to follow, try this template: www.find-the-words.com