I honestly never know how to answer this question, because I'm still not really sure how it happened myself. It was definitely God's doing, because I don't think I was smart enough on my own to know that a mission was the right path for me.
It all started when I was organizing my schedule for Fall 2012. I needed a religion credit, and I heard that mission prep was easy. Therefore, though I had no intentions of going on a mission in the near future (how could I, I had just turned 19), I enrolled in the class.
Conference comes along. I was supposed to work an 8 hour shift on Saturday, which sounds horrible but I was actually really happy about it because I needed the money at the time. I could watch Conference online later, I reasoned, and all would be well.
BUT THEN, the week before Conference, I got the strongest feeling that I needed to watch Conference live on Saturday. For some reason, I felt like it was absolutely essential. So, despite the financial loss I knew it would incur, I got my shift covered and headed to my parents' house to watch Conference.
The moment that President Monson made the announcement that young women could serve their missions at age 19, I just knew that I was supposed to go. I decided with no hesitation that I was going to start my mission papers immediately, and that's what I did. Five months later I was in the MTC.
So, it was kind of a strange path that lead me to decide on a mission. A part of me wishes I could say that I had always wanted to go, that I had studied it out, that I had prayed about it, and that I then received an answer. Well, that's not the way it happened, but I don't think that's what is most important.
What is most important is that I went and that, for me personally, it was what I really, really needed to progress. I can't think of anything that fills me with more gratitude and love for my Heavenly father than my mission. I could go on and on about all the good that came from the decision (which didn't even feel like my own decision, really) to serve a mission. For me, it was the push I needed.