Just google what you are interested in. In my opinion, you don't need an app. There should not be an app for everything.
You have to get out, travel around, see what is growing in your neighborhood. See what you like.
Go to plant nurseries, big box stores and see what they are selling. Then once you have a few ideas of what you like, then you can just 'google' something. A lot of good information pops up.
But gardening is getting down in the dirt, get your hands dirty, feel the soil, look at the plants, their structure, their flowers. There is no app for that.