You're asking multiple questions, each of which should be their own separate questions, but I'll answer them both here, anyways.

(1) (BACKUP ANY DATA YOU WANT TO KEEP BEFORE DOING THIS!) To change the Operating System, the VPS provider should be able to do that. If there's not an automated "Reinstall Operating System" option in the VPS control panel pages for your provider, file a support ticket with your VPS provider. If they tell you that you cannot switch operating systems, or that they do not have Ubuntu VPSes, then you won't be able to switch to Ubuntu (but most VPS providers have Ubuntu images)

(2) Once the VPS is set up with whatever OS, you should SSH in by specifying the IP address for your server, then it'll connect and prompt you for the username and password. Provide those, and once you're logged in, do sudo useradd --create-home --user-group --shell /bin/bash USERNAME where USERNAME is the login name for the user you're creating, because the default user you login to will either be root or a user with sudo access