Well, I'll be honest with you- no one can ever be completely prepared for a mission. But that's good since you wouldn't learn anything otherwise. The mission will also be the hardest thing you've ever done (at least it was for me), but it is more than worth the hardship. You will learn to be selfless; nobody ought to serve for their own benefit, though you do receive many blessings.
However, you should at least be somewhat prepared. President James E. Faust gave a great talk on preparing for missionary service which you will find here. Though it's addressed to the men, it can just as easily be applied to the Sister Missionaries: http://library.lds.org/nxt/gateway.dll/ ... ission.htm
I want to add to what he said about avoiding the mysteries. Focus on learning the doctrines of the missionary lessons. (Also, get Preach My Gospel if you don't already have it.) As far as applying doctrines are concerned, focus on the first principles and ordinances of the gospel.
Many of the people here or anywhere else who have been disaffected by the Church became so because they strayed from these principles- thereby losing their spiritual foundation, their "rock"- and got caught up in vague teachings and alleged statements/opinions of past Church leaders. Now if they had kept their testimony and understanding of the basic principles strong, they might have been able to comprehend some of the deeper things by the power of the Spirit (or would have discerned some of the other things to be opinions- and not the doctrine of our Heavenly Father). However, it can be applied to them what Peter said of those who could not understand some of Paul's writings:
"As also in all his [Paul's] epistles, speaking in them of these things; in which are some things hard to be understood, which they that are unlearned and unstable wrest, as they do also the other scriptures, unto their own destruction.
"Ye therefore, beloved, seeing ye know these things before, beware lest ye also, being led away with the error of the wicked, fall from your own stedfastness.
"But grow in grace, and in the knowledge of our Lord and Saviour Jesus Christ. To him be glory both now and for ever. Amen." (2 Peter 3: 16-18)
That's all I can say. Pray about it. Know that you have a testimony. Seek counsel from your parents, bishop, and stake president; seek their counsel more than any other person's.
I can't give any advice about physical and other aspects of preparation because I don't know your situation. Good luck to you, even if you end up not serving a full-time mission.