First login to your VPS

SSH in

Your welcome email has the IP. SSH as root with the password in that email:

ssh root@YOUR_IP

Add an SSH key, disable password login

Create a user, paste your public key into ~/.ssh/authorized_keys, then disable password auth in /etc/ssh/sshd_config:

PasswordAuthentication no PermitRootLogin prohibit-password

Reload sshd. Keep your original session open until you've tested the new one.