You've done 2 of the three steps. If you want www, always ensure www.domain.com is listed in the site definition and domain.com in your alias (as you have) and not the other way around. The last step is to go to Settings>URLs & SEO>SEO - URLs>Process domain prefix and set to "always use domain with WWW prefix". This will force your domain to use WWW.
Keep in mind if you're still in dev, and have a subdomain you'll want to set this to "leave domain as is" otherwise you'll never get to you subdomain.