Most missionaries I asked just said read preach my gospel and the book of mormon.
1)But if you can read the new testament, d & c and the pearl of great price.. because from there is where we learn about pre mortal life which we have to testify about often.
2)Also Check out the website "fair lds" because you will no doubt have people tell you on your mission the church is false and this is why "blah blah blah" but if you know some of the things they will say and how to defend yourself you will not have doubts. On your mission you cannot take "apologetics" books (religious defense ) so just really get to know the scriptures by looking at them.
3) pray for further testimony even if you have one. If you start without one people you meet at the start of your mission may loose out on the spirit.
4) get into healthy scripture reading habits and mark them. but dont close them straight afterwards... leave time to pause and think "what does god want me to learn from this".
hope you get you call soon and everything goes ok.