Where can I see the rewards I will get from the event?

You will want to tap on the portal, then on the rewards button (the chest icon). You can view the rewards for each leaderboard tier here.

The highlighted tile with your amount of trophies displayed in the top right corner corresponds to your leaderboard tier.

Once the event has started and you have started earning Ninja Coins, you will be able to see the rewards you will earn here.