All currencies you transacted will be automatically followed when you enable Price Alerts, still, you can personalize the currencies you're following.
1 - Tap "Markets" at the bottom of your home screen
2 - Select your currency
3 - Tap the star at the top right corner to follow or unfollow a currency
4 - When you're at the "Markets" page you'll see a yellow star near the currencies you're following
You can see all the currencies you're following at the "Markets" page under "Following".