Hello,
It is recommended to have an understanding of Java or Python as most of the codes are written in these languages. Also , you will need a Google Cloud Platform account. If you don't have it already, you can sign up for a free trial account which will be valid for a year and you will be able to run up to $300 worth of services.
Also, I would recommend you to watch introductory videos [1],[2] to have initial understanding of Google Cloud Dataflow.
[1] https://www.youtube.com/watch?v=1l-PBRothn0
[2] https://www.youtube.com/watch?v=KalJ0VuEM7s