Mobile app Questions

I want to build an app and have a great idea! but how do I know it will be possible?? And what about if other people have built it already how can i go and search for apps that are already on phones? Its an android app that I want thanks

The quickest way to see if an app like yours already exists is to search for it on either the Apple Apps Store (in the case of Apple apps) or Google Play (for Android apps). Since most people don't have both kinds of smartphone, you can always use Google or Bing to pull up other apps in the same category as the one you're proposing (such as "car repair app"). Such a search will tell you pretty quickly if there are already similar apps out there.

One more note: if you find there are other apps similar to the one you're wanting to create, it's not necessarily time to throw in the towel. There's always the possibility that new and unique features could be added to make your idea competitive in the app marketplace.