Hi Sharon Huang
Thank you so much for posing this question on GeoNet. We appreciate your eagerness to dive into GeoEvent Server and work with/learn all of its capabilities.
However, I am sorry to say that Esri does not provide any publicly accessible GeoEvent Servers for testing. This product is really meant to be managed internally as the services that you create from it oftentimes require that you provide access credentials to specified endpoints (JSON/XML feeds, feature services on AGS, etc.). GeoEvent will work to process incoming data and then output it to things like web services, local files, emails, and the like. Because of that, it would not exactly be feasible to allow the public to all access one GeoEvent Server.
If you would like to discuss product licensing and pricing for your organization, I would encourage you to reach out to either your account manager or Esri Customer Service and they would be able to help you get started if that is something you think can eventually be implemented for you and your team.
I hope this helps.