To develop an application for watchOS, you should start with Xcode. Xcode only runs on macOS. At the time of writing, the latest version is Xcode 8.3.
If you want to start a new project from scratch:
Boot up your Mac and install Xcode from the App Store if it's not already installed.
Choose to create a new project.
In templates, choose watchOS and then "iOS App with WatchKit App".
If you already have an iOS project and want to add a watchOS target: