You want Ubuntu 16.04, the install instructions for discourse are in:
discourse/INSTALL-cloud.md at master · discourse/discourse · GitHub
You also need to decide what email provider you want to use, and get that setup before you run
./discourse-setup , as that will ask you for information about your mail provider.
My advice is even if you get all the way through the install and everything works, play with it a bit i.e. create some categories, and some posts, and a couple of users, and get used to how updates, deletions, backups and such work, including the CLI commands and shutting down and restarting your server. Take notes about what commands you ran, and what they do.
Once you’ve familiarized yourself with the basic admin tasks, then start again from scratch before you start inviting people and go live with your forum.