You can get a free trial here or here.
Edit:
If you’d like to charge for access to your forum, you’d need either the patreon plugin (which I do not believe is included with those free trials) or to do some custom work to connect, e.g., some WordPress membership manager to Discourse groups via the API.
I think that what you want to do is to try out one of those and then you can make plans about moving forward with figuring out the Membership stuff.
Another option for a free trial would be to use the $10 free Digital Ocean referral link on my discourse install page and follow the INSTALL-cloud instructions.