For your domain to be accessible using
www.domain.com, a few things need to be setup. The first is your DNS and the second is your web server.
Your DNS is the easiest, so we can start there. To get started, login to your domain registrars website or, if you're using DigitalOcean to host your DNS, login to DigitalOceans Control Panel and navigate to the Networking page and click on your domain to pull up your DNS records.
Once ready, you should have the following entries.
A domain.com DROPLET_PUBLIC_IP
A www.domain.com DROPLET_PUBLIC_IP
domain.com with your real domain and
DROPLET_PUBLIC_IP with the IP address of your Droplet.
Once your DNS is setup, you need to double-check your web servers configuration and make sure it's configured to accept both. The configuration will differ depending on whether you're using Apache or NGINX.
For Apache, your VirtualHost block should look like:
For NGINX, your Server Block should look like:
server_name domain.com www.domain.com
Of course, the above is just snippets and not full VirtualHost or Server Blocks. Simply make the needed changes to the lines that correspond to yours.
Once the above changes are made, you'll need to restart Apache or NGINX:
service apache2 restart
service nginx restart