Where do I download it, and how do I know it is the real app?

Only from the App Store or Google Play, published by Diverse Inc., the company behind Creta. Follow the store link on an official Creta site rather than searching. Search results and adverts can show look-alikes.

Never install a Creta app from a file someone sends you, or from a link in a direct message. A fake wallet app has one purpose: to capture the seed phrase you type into it. If you have entered a seed phrase into an app you now doubt, treat that wallet as compromised and move everything in it to a new wallet.