Take a look at this article

https://www.digitalocean.com/community/articles/how-to-set-up-ssh-keys--2

Personally, I'd recommend this tutorial: https://library.linode.com/securing-your-server - it's very detailed.

THank you!

Hi,
I started the tutorial (digital ocean) and I was able to get to this point
cat myfiile_rsa.pub | ssh myusername@1.1.1.1 "cat >> ~/.ssh/authorized_keys"
myusername@1.1.1.1's password:
bash: /home/myusername/.ssh/authorized_keys: No such file or directory

Do I have to create the directories? install a package ?

Note I replaced my ip and username with fake ones (1.1.1.1 and my username )

I created the directory .ssh and now it works...