For a simple button, I would suggest the “On / Off Button Tile” sample device type that you can find by creating a new Device Type and browsing Device Type Examples from the coding screen.
For the Restful API, I’m assuming you mean for something to make REST calls to SmartThings? If so, this thread has a simple example. Everything else I found was mixed in with a lot of code.
If you mean to call a REST api, the docs look pretty straightforward:
https://graph.api.smartthings.com/ide/doc/deviceType
There are probably some good examples as well, but I haven’t explored that area yet myself.