Of course we start with Hello World. Follow the connection instructions, then click on the left hand side of the IDE and type:
Now stop and think for a minute about what just happened there in terms of communications flow.
You should try out other aspects of the Web IDE. It has full auto-complete in the REPL window including Button/LED names and functions.
You could also try the built-in Blockly graphical editor which is very similar to Scratch.
Any Espruino code you find here or elsewhere online can be pasted into the editor on the right-hand-side of the IDE and then sent to the badge over Bluetooth for execution. The editor has basic parsing and can flag common errors.