Many good and very fine answers to this!! For myself,I was told to start in the book of Romans,the reason? Because the Book of Romans shows us first where we are presently, the book shows why God and his righteousness is better then our own.( rom 5) how to apply this verses 12-21) Chapter 6 points out because of this grace we now have, is this a reason to sin more? Chapter 6
Then just when we think we are the only ones fighting the problem of our own flesh, Paul writes what every believer needs to understand within themselves!! WE ALL struggle (Rom 7:15-25) Then our truth in this struggle!! chapter 8. What is expected of us,how we are to live. The battle for every believer starts where we have the greatest problem! In our own minds!! ( rom 12:1-2)
The Word is to take the place, of our fleshly mind. We have to make a new tape in it.And this only comes by digging deep into the Word of God! To replace I cannot, with I can!!( rom 8:37-39) That nothing will ever come between God and me!! When you have this done!! THEN!! You will have the confidence you need to press forward,never looking back,to see that great example for us!! Jesus!!( the gospels!) John is my favorite of them as well.
I hope I have helped you,we all can start from wherever we wish,but the point is!! to start! For one can never win a race,until one is in it!( 1 cor 9:23-27!!) It takes discipline!! Something our flesh hates with a passion to do! Blessing to you all in Christ!