First, a caveat: I'm new to Digital Ocean. But the following works on my other VPS, and I don't see any reason why it shouldn't work here:

[from nginx.conf]
server {
listen 80;
server_name mydomain.com www.mydomain.com;
...

or, if you want to use only the www.* form:

server_name www.mydomain.com;

If I'm not mistaken, there is no redirection when you do it this way (regardless of how many values you specify for server_name).

Of course, you also need to make sure your DNS record includes the www.* form.

Oh, wait. Do you mean you really want a redirect, such that if a visitor types in 'domain.com', they will be redirected to 'www.domain.com'? My response doesn't help with that, it only enables the server to handle the 'www' form (or both forms, depending on which version you use).

Thanks @matt.

I've already added www to my DNS.

if I added www to the domain in the etc/nginx/sites-available/mydomain, not at nginx.conf in the nginx root folder.

That works, but counted as an extra request. so an empty page takes 2 requests, which is a bad practice.

I believe there a way how to do that, but not sure yet how!!

regards

Oh, I see, it's one of those convoluted Debian configurations. I'm not saying "don't use Debian," but I think sometimes they make things much more complex than they need to be.
Hopefully some other Debian user will come along with a definitive answer. However, if you want to go exploring, my method should allow your server to handle the hostnames you want, without any redirects. Nginx configuration is pretty straightforward--if you find the main config file (I checked the Debian package DB, and there is an 'nginx.conf', so that should be it), it should contain one or more 'include' directives; if you follow those to whatever file contains the 'server_name' line, you should be able to make the change there. Best of luck with it!

@matt, you are right. I guess there is no way around this anyways. I've been searching on the net for almost a day and got nothing, but only using proxy_pass or rewrite module through the site-available domain file config.

and here is the solution to redirect from mydomain.com to www.mydomain.com:
-------------------------
server {
listen 80;
server_name mydomain.com;
return 301 http://www.mydomain.com$request_uri;
}

server {

listen 80;
listen [::]:80 default_server ipv6only=on;
server_name www.mydomain.com;
}
--------------------------------
Thanks again.

Regards,