Be a Missionary Now
You expressed a desire to become involved in missions. You have to start now, right where you are. This is very important in preparation for missions because the best missionary at home will be the best missionary overseas.
What kind of activities do you anticipate doing as a missionary? Find ways to develop those skills now.
If your desire is to work with kids, for example, you might want to help out with Sunday School or kids' clubs. You might find that your best first move is taking more of a leadership role in your own church, youth group, Campus Life, Young Life, or other activities you're already involved in.
Talk to your pastor or other spiritual mentor about ways you can serve as a missionary. Do research. Seek out opportunities to be a good missionary in your community.
Be a personal evangelist—don't hide your faith altogether, but don't make it the basis for every friendship either. Make sure you have some non-Christian friends and that you're a real human Christian example to them. Love them and share your faith with them through your actions...and then through your words when they start asking why you do certain things.