How do I reset my Droplet's root password?

You can reset your password using the DigitalOcean Control Panel.

After resetting the password, we will email the new root-level password to your account’s email address. When you log in to the Droplet with your new password via SSH, the operating system will ask you to change the password. It will first ask for the current password (the new root password emailed to you), then ask you to enter the new password twice.

You may be receiving this error for various reasons, including a missing SSH key or incorrect password.
You can review disk usage on your Droplet and then remove unnecessary files.
There are a few ways to install an SSL on your Droplet, depending on where you get the certificate from.