Quite a vague post you have there. You don't explain what you mean by "app" so I will go with the most common usage and assume you mean a phone app.
Basically two directions to go from there: iOS or Android. Now, it's safer to assume you have PC over an Apple, which means you are probably going to want to go with Android.
For Android, it's pretty simple. Go to Google's Android developers page [http://developer.android.com/index.html]. Lots of resources and easy instructions to get it set up.
If that's not what you want, then maybe think about being a bit more clear with your thoughts before asking for help.
