Your question isn't overly specific, so it's difficult to give you guidance. You can use the REST API with Tableau Online, although not all the methods are available with Tableau Online as are available for "full" on-premise or cloud hosted Tableau Server that you manage.
The best place to start is with "help":
Keep in mind that the REST API isn't really about pulling data out of Tableau Server - it's about managing the server itself. If you can be more specific about what sort of data you need to get from Tableau Online, we can probably give you more guidance.
I am also trying to gain access to Tableau's REST API and I am wondering what I should use for server. Is that simply online.tableau.com?
My aim is to send automated emails containing images or PDFs from dashboards. Don't know if REST API is the best way, but I can not locate Tabcmd in the online environment.