The easiest way to get a comm is be an experienced player, but the polite one.
Like in dungeons, trials or raids you can give advices first before the fight starts. New players will think that you're kind enough to give advice politely (not by insulting them because they're bad at first run)
Or as a healer (that's the fastest way for me) you heal and perform a little DPSing by giving DoT. Use Rescue when a player got stuck and unable to move. Casting HoT like Medica 2 will help too.