To start with App Development we need Visual studio 2013 or higher version. Download latest community or expression edition from here > https://www.visualstudio.com/products/free-developer-offers-vs
Once it has been downloaded and installed
Open and Click create new project
expand Office/SharePoint section you should see an option for App as shown below.
If App option not available Close the VS , download and install Microsoft Office Developer Tools https://www.visualstudio.com/en-us/features/office-tools-vs.aspx