Dear Jeni,  

The call you are feeling is still in many respects vague, but since you are doing all you can now to find out what you should do concretely, I am sure that as soon as the right thing crosses your path you are going to recognize it. Have trust! I will send you in a separate message some concrete information that may or may not be the way for you to go, you can be the judge of that, but here are a few general things you can keep in mind and may be of help also to other readers.  

One, you don't have to wait until later to be a missionary. You are called to be a missionary among your friends and peers right from now. Can you start and inquiry group into the faith? Or an "apologetics" group? Or organize some of your friends to do charitable work (help out in a soup kitchen now and then, visit a children's hospital, do a food drive...)? 

Usually doing missionary work means getting involved with some group or other that already does this type of work, is organized, that can give you training and support, etc. So it would be good to start looking into the groups you know of, or looking up new ones.  

Three, missionary work comes from the inside. The real power of missionary work is that it comes from the love we have in our soul for Christ and our desire to serve him in others. So right from now try to grow in your prayer, tell Christ you want to do his work, that you want his love to guide you, that more giving people material things you want them to experience God's love for them.  

I'll keep you in my prayers. God bless.