To find apps, begin by launching the App Store from the homescreen of your Apple TV. Once the App Store is open, there are four ways to find apps:
Featured Apps—If you've used the App Store on the iPhone or in iTunes, the layout of this App Store will be familiar to you. The main screen includes a series of featured apps at the top and then collections of newly released apps, popular and top-selling apps, and apps selected by Apple for promotion. Use the remote to navigate this screen until you find the app you want to install
Top Charts—Select the "Top Charts" category in the menu across the top of the screen (if it's not displayed, swipe all the way up on the remote's touchpad). Once you do that, you can view apps in ways like "most downloaded", "biggest sellers", "highest grossing", etc.
Categories—Go to the options in the menu at the top of the screen. You can browse apps in categories like Games, Education, Lifestyle, and News
Search—The final option in the top-level menu. Select it and use the onscreen keyboard to type in the name or type of app you're looking for.