How to set and use ssh key for passwordless login

Hi, I have following question. How to set passwordless ssh login (ssh with passphrase) so I could login only if my key was in authorized keys file?

I am using OVH Public Cloud instance and there they permit to login via ssh key only (there is no password generated for user). When you login and must do sudo it is enough to sudo with -i argument and you're root (no prompt for password). Of course you can set your passwords and stuff but by default they use those settings.

Of course another question is if this is safe?

Thanks!