Excerpt from a great little booklet called "Missions and You!" by Larry W. Caldwell:

Many of you may be seeking to determine God's will for your life in relationship to the Great Commission.

Here are some helpful questions to ask yourself to see where you might fit in.  

For what part of the world do I have an interest or burden?  
From the time you were a small child you may have had a special twinge inside whenever you heard of a certain country or people group.  Could that feeling be from God?

What has God already spoken to me about missions?  
God rarely uses lightning strikes to convince somebody to become a missionary. Instead He uses His "still small voice" that gently leads us towards doing His will.  It is important to not only listen for that small voice but to be obedient to it.

What type of missions ministry might I fit into?
It is always a good thing to know what are your gifts, abilities or skills, especially if you might be considering missions.  Believe it or not, God can use just about anybody as long as they are willing to be obedient to Him.

What is my "calling"?  
How does this sense of a calling relate to my gifts, abilities and skills?  Not every one is called to be a Billy Graham, nor a WilliamCarey.  It's important to match your calling with those gifts, abilities or skills of which you are already aware.  Nothing magically happens once you board the airplane, train or boat bound for the missions field.  Start by using what you have.  It's what God gave you.  Do I have any negative attitudes toward people of other races/cultures?  Negative attitudes don't miraculously disappear once a missionary gets to the missions field.  All Christians need to work on these negative attitudes, but especially potential missionaries.  If these negative attitudes are too strong you may not really be a good missionary candidate.