Download and install Node.js.
Install Monaca cli using the command npm install -g monaca
Create a new project called "MyOnsen" using the command monaca create MyOnsen
Once the installation of Onsen is successful, you would see a project folder MyOnsen. Now open MyOnsen folder and navigate to app.jsx file in src folder
You can start coding your application in app.jsx.
Once your app is ready, the same can be previewed in the browser by typing in the command cd MyOnsen monaca preview
To review the app in any mobile device, use the command monaca debug.