In order to be an effective missionary, it is helpful to have an understanding of cultural anthropology, missions history, missions strategy, and the theology of missions. There are several places where you can get a good foundation in these areas:
A good starting point for learning the basics of world missions is a course called Perspectives on the World Christian Movement which is offered in various locations by the U. S. Center for World Mission. The course gives an overview of missions through readings, missions speakers, and tests and papers (if you take it for credit). You can get certificate, college, or seminary credit for the course, or you can simply audit it. The work you put into the course is definitely worth it.
If you have not yet completed college, you may want to consider attending a Christian college. Such a school integrates foundational courses in Bible study, theology, and Christian living into the academic program. These schools offer a variety of majors, and you could even choose to major in World Missions or Intercultural Studies to prepare you for the mission field. For a directory of Christian colleges, see the Christian College Handbook published by Berry Publishing.
Keep in mind that secular colleges or universities are also a good option, providing you with the opportunity to be a witness among your peers while earning a degree. Most colleges and universities have Christian groups such as InterVarsity, Fellowship of Christian Athletes, Navigators, or Campus Crusade for Christ on campus. These groups help provide you with a Biblical foundation in the university setting and offer training in evangelism. I attended a secular liberal arts college and became involved in InterVarsity there. During my college years I realized that God really was calling me to involvement in missions, and then I decided to attend seminary.
If you have completed college, you might want to consider attending seminary. Some missions agencies require you to have at least a year of Bible courses under your belt before you sign go overseas, and seminary is a good place to take this coursework. Seminaries vary in the courses of study they offer. Nearly all offer a (3-year) Master of Divinity degree which most churches require for ordination. If you wish to be a pastor on the mission field, this is probably the degree for you. Some seminaries offer a (1-2 year) Masters degree in Missions. Such a program usually combines theology, Bible, and missions coursework with a missions internship of some kind.
Another option is attending a school or training program that specializes in preparing you for missions, such as the Christ for the Nations Institute. This kind of program emphasizes Christian discipleship and missions training and takes less time to complete than College or Seminary.
Check out the Urbana Educational Exhibitors for an online searchable index of various educational programs.
As you examine your options, keep your financial situation in mind. Try not to go too far into debt because enormous student loans can delay your trip to the mission field. Find out what scholarships are available and apply for every one that you can. Will your church help you with tuition? Find out. Some missions organizations may allow you to raise money to pay for student loans as a part of your support, depending on how far into debt you are. Others will not.
I was fortunate that my parents covered my college expenses. When I attended seminary I applied for scholarships, work study, and financial aid, and I took out a Perkins loan with a 5% interest rate. My church also helped pay for my tuition. Be careful not to leave financial considerations out of your prayers and the decision-making process.