I've been told that each one of us has a specific, individual mission in life. How do I know what mine is?
King David writes, "The steps of man are directed by G‑d" (Psalms 37:23). The Baal Shem Tov, the eighteenth century founder of the Chassidic movement, explained this verse in the following manner: Although we go about our daily tasks at what seems to be our own initiative and will, our steps are "guided" for a spiritual and loftier purpose. We end up in a specific place so that we will have the opportunity to do a mitzvah (good deed or Torah precept), help someone in need, or to pass on a positive message.
In short: You don't have to worry about finding your mission. G‑d will direct your steps to the location where you mission lies; He'll orchestrate the circumstances to ensure that you have the position and tools to execute your mission. The challenge is to seize the moment and utilize every opportunity you have to do more good. When you find yourself in a specific place and situation, that itself is the greatest indication that there is something for you to accomplish there.
Another important hint: it is specifically the tasks that you find most difficult to carry out in which you need to place the most effort. Because true achievement only comes through challenge, there will be many more obstacles and challenges placed in your path when a particular assignment is the one that you, specifically, need to accomplish...
In conclusion, allow me to suggest some additional reading on this subject. You can start by browsing our knowledgebase's entry on What's my mission in life? I particularly recommend Diamonds and Fish, a very instructive parable, and There Now, a vacation meditation (video). You also might want to check out some articles and stories about the Baal Shem Tov.