Answer from Nate, who served as a missions mobilizer with Caleb Project.
Maybe you don't have a degree, but there's probably something you know a lot about and that you do well, whether it's motorcycle repair or worship music or serving. Whatever it is, God has put those specific tools into your hands to use for his glory among the nations. With a little knocking on doors, you can find a niche somewhere in world missions for anything you're good at. One warning, though: You shouldn't become a missionary unless you're willing to learn new things in the process!
"No matter your role, some training is necessary."
Answer from Jack Voelkel, missionary-in-residence with the Urbana Student Mission Convention; originally published on the Urbana website. Previously Jack served thirty years with Latin America Mission in Peru and Columbia. Find other answers and articles from Jack and others on the Urbana blog.
If you're thinking of working in missions, you should first look for an agency to send you. All agencies have requirements, which may include educational preferences. Missionaries do many things today; not all are preachers or teachers. All should be prepared, however, as Peter tells us (1 Peter 3:15), to "give an answer to everyone who asks you to give the reason for the hope that you have," whether working as a physician, agronomist, or mechanic.
Our ministry, wherever we are, should be holistic. Thus, the proper perspective would be to decide what the Lord is leading you to be and do, and then, as much as possible, fully prepare. If you get formal training, make sure you include some courses in cross-cultural communication.
"We are just ordinary people."
Answer from Richard, who has served with Commission To Every Nation in Texas and Guatemala for twenty-four years.
I direct an agency that sends people around the world - currently fifty-one nations - who are just ordinary people. There are innumerable opportunities for people who are willing to serve behind the scenes, be an Aaron or Hur to lift up the arms of tired leaders. The harvest is white, the laborers are few. I believe one reason the laborers are few today is because we make it so hard to go - implying missions is for special Christians, ones who walk on water and glow in the dark. It just is not so. There is a place for you.