Agreeing Mabel that people have all elements to them - the idea that a person is of one element appears to be primarily from fictional games and more recently from that Last Airbender cartoon. Many people want to know their elemental alignment so they can focus on that particular element (again mimicking the fictional worlds in which characters have gifts and powers of a particular element), but I will actually suggest the exact opposite. I have a lot of Fire compared to other elements, and while this has positive sides like meaning I have a lot of passion and energy and am not a pushover in any way, it also means I am prone toward anger, leaping before looking, and I suck at holding still long enough to heal from an injury. Likewise I have an ex who is equally unbalanced toward Air, and while this means he is incredibly thoughtful and able to understand incredibly complex ideas, he also lives in his head and isn't very practical (which belongs to the opposite of Air, Earth). So please don't take your elemental leaning and concentrate on that element so much you become even more unbalanced. Make use of the talents it grants you, but be sure to counter for any unbalanced tendency you have toward any given element.
So how did we discover these elemental tendencies? By looking at our astrological natal charts. What most people think of as "their sign" is only their sun sign - you've got around a dozen other things in their own signs, and your Sun sign may actually be one of the only things you have in that particular element. You can make a natal chart on astro.com, and feel free to PM me if you need help understanding it - Astrology is one of the things I chose to specialize in when I joined OTO.