Pick any - everyone will receive a prize, but it seems that WHAT the price is depends on the winning team.
You may need to join a team first for the event to properly start. Join a team, then browse forums. Look for an orange button underneath people's avis. Click it. You'll earn points for your team (between 1-3 points), and one gem. Exchange the gems for items.
When your HP reaches 0, the buttons stop appearing. Each click consumes 5 hp. Posting should regenerate some HP in some way, but we're not seeing it yet. HP recovers every 24 hours. You can see your HP and points in the teams page.
I think wearing some outfits/outfit combos will increase stats? I only got one item so far, so I can't confirm.