That is exciting! 3-5 minutes is plenty of time. 

Some numbers that would be helpful to have on hand:
- How many monuments are there in your country
- How many of those still need a picture?
- How many pictures have you collected in the past year(s)?
- What can people win nationally? Internationally?
- How many countries are participating? (48, spread over six continents)

Also good to have clear:
- Why do you personally organize this competition? (for example: because I'm proud of my heritage. Because I want to introduce more people to the joy of editing Wikipedia. Because I hope to diversify the community)
- Why should people participate? (for example: because it's fun! to explore local heritage. Because of the prizes (for great photographers), or because they can help Wikipedia (for amateurs).)
- How to participate? (for example: For Denmark, go to wikilovesmonuments.dk and for other countries go to wikilovesmonuments.org . Find a monument to photograph, create an account, and upload the photo.)
- One clear example of a monument site you were excited about to see an image of. What image touched you? Make sure this is not a 'big monument' but rather an unknown building, but something that people can relate to.

Have one clear throw-out quote ready (10-20 words) that you can say at the very end. Something along the lines of "If you want to help share Danish heritage, go to wikilovesmonuments.dk and upload a picture of a monument nearby you."

If you want to do a bit of reading for inspiration for quotes: 

This article from Australia looks nice, or this from Ireland or page 170 of the Heritage in Motion. 

Hope that helps. 

Lodewijk