March 4: D&C 4:5

5 And faith, hope, charity, and love, with an eye single to the glory of God, qualify him to the work.

Okay so now that you've been called to the work, and understand that the work (in this case missionary work) has marvelous blessings, this is what actually qualifies you to go. Have faith in the gospel. Live each day fully commited to the Lord, he knows what is best for you, and will bless you beyond anything you can imagine when you follow his teachings. Have the pure love of Christ and follow the Holy Ghost. Living with the missionaries, I can really say that the effective missionaries I've seen are the one's who try and live the qualifications presented in this scripture.