How can a solider in the middle of a war be effective if he or she doesn't know what their assignment is? Are they supposed to lie back and defend or should they attack? They'll end up not doing anything, the wrong thing or even end up hurting soldiers on their side.
When it comes to the Christian walk, which God refers to as a spiritual battle (Eph. 6:10-20), many Christians have no idea what their assignment is. This reduces the Christian to an ineffective life. This Christian ends up walking aimlessly around the battlefield in the wrong place and maybe shooting Christians along the way (which happens way to often). If there's enough Christians who are like this, the entire army is reduced to ineffectiveness.
So how does one determine what one's calling is? First we have to discuss that there are general callings and specific callings. General callings are for all Christians such as to worship God, be a good witness, and transform culture. But God has given everyone specific callings - callings personalized just for you.
God has given you three main things to figure out what your calling is.
#3. Resources It can be put this way: Gifts + Talents + Resources = One of Your Callings.
Gifts are spiritual areas of life that God has given you (Rom. 12:3-8, 1 Cor 12). Some gifts are: teaching, service, preaching, encouragement, generosity, music, hospitality, etc.
Talents are physical areas of life that God has given you that you are good at. Public speaking, singing, mathematics, debate, accounting, writing, cooking, interior design, child care, etc.
Resources are physical objects God has blessed you with that enhance your gifts and talents. Money, car, house, friends, schooling, degree, church, family, etc.
For example God has called me to be a teacher of doctrine. When I was in high school I was learning that I was getting pretty good at teaching and logic. This was an inkling that I might have a gift in this area, but I wasn't sure yet. Later in college my public speaking (a talent) increased. I started to learn more about Christianity in college and continued in Seminary - these are resources. The gifts, talents and resources put together point me to my calling of teaching doctrine.
Another calling God has given me is: giving money to his work. God has given me the gift of generosity, talent in accounting, and the resources from that job would be money. Those things combined make me realize that I have been called to be a source of income for God's work. So if my father's ministry in the Philippines needs money I am quick to give. If my friends need monetary help in a rough period of life, again I am quick to give. My gifts, talents and resources point to my calling of giving.
If I didn't know that this was a calling I wouldn't be so willing to give, and even if I did give money, I would not be worshipping God in my service. Furthermore the ministry in the Philippines would have a lot less money to work with - lessening its effectiveness. So me not knowing just one of my callings affects potentially hundreds of people in two countries.
God is telling you what your calling is by these three things. You can't wait for God to tell you what your calling is, because he's already telling you what it is through your gifts, talents and resources. That would be like waiting for God to tell you the answer when it's already right in front of you. You ask, its there, you wait, its still there, you question why you haven't received any direction from God - it's been there the whole time. To 'hear' God, you must determine your God given gifts, talents and resources.
God has a calling for everyone. Sometimes you are at a point in your life that you do not know your calling for when you are an adult, but you still have a calling right now. For most kids the calling is a little more general and becomes more specific as life goes on. God has called most everyone to be a student early on. I wouldn't have been able to be as effective as an accountant to make money for God's work now if I screwed around in high school and college and got bad grades, got a useless degree and worked at Spatula City making minimum wage. So for those of you in school right now, that is one of your callings - so do it well. God is preparing you for a more specific calling that will have wide ranging effects.
Figuring out your calling isn't the easiest thing in the world to do. For more help on how to do this go to the article entitled, "How Do We Discern God's Will?"