Romans 8:12 Therefore, brothers, we have an obligation—but it is not to the sinful nature, to live according to it. 13 For if you live according to the sinful nature, you will die; but if by the Spirit you put to death the misdeeds of the body, you will live, 14 because those who are led by the Spirit of God are sons of God. 15 For you did not receive a spirit that makes you a slave again to fear, but you received the Spirit of sonship. And by him we cry, "Daddy, Father." 16 The Spirit himself testifies with our spirit that we are God's children.'

I have been a Christian for a long time... and I have a little biblical education... but even with all that, a passage like this is so full of depths of significance that it is still difficult to grasp.
To put things as simply as I can:
We DO have a sinful nature. We do desire things that are contray to the will of our holy God. Furthermore, we are very motivated to follow that sinful nature into all kinds of sinful practice. Furthermore, we are powerless in ourselves to resist the cravings and the actions of our sinful nature.
Through faith in Jesus, God fills us with His own SPIRIT.
This Spirit brings with it some VERY significant changes.
We go from a relationship with God that is focused on what we DO... ie: sin or don't sin... and into a relationship with God based on WHO WE ARE... ie: Son's of God.

No more is it about do this or do that.
Now it's about DADDY!

When I employ a man to empty my trash bin, the relationship is one of works. He either does the job or he loses it. If I come out and find my trash bin has not been emptied, then he will get no pay and I will not call upon him to work for me again.
When I give birth to a son, the relationship is one of care, love and delight. I will take him into my arms and adore him with all of my heart. He will run to me and cling to me and cry out to me, "Daddy, Father!"
It will be from within that relationship of Father and Son that I will teach him, and disciplin him, and work on shaping him into the man I wish him to be.

This Daddy, Father relationship with God is what the Holy Spirit brings us into. If we follow His leading, He will teach us to DO those holy works we could never do when it was our legal obligation to do them. No longer do we seek to be obedient by following rules, but we obey as a result of a loving relationship.

So how do we get there?
"Be filled with the Spirit" (Eph 5:18).
Seek relationship with God, pour out your heart to Him in prayer and surrender your all to Him. Ask Him to fill you with His Spirit and to lead you by His Spirit. Draw inspiration from His Scriputres. Be filled with Grattitude ("Eph 5:19 Speak to one another with psalms, hymns and spiritual songs. Sing and make music in your heart to the Lord, 20 always giving thanks to God the Father for everything, in the name of our Lord Jesus Christ.")

A son who feels loved by his Father, and who in turn is greatful to his Father, is a son who will walk in close relationship with him. Such a Son of God will be a loving, holy, man.... it will just happen.... it will flow quite naturally from the relationship. The apple doesn't fall far from the tree.

Prayer
Father, dear Father, DADDY!
Please fill us with your Spirit. You have already given us so much in sending Jesus to take the penalty for our sin in our place. He did this so that we might recieve Your promise - Your Spirit. Fill us please and may our hearts be filled with songs of grattitude. May we know you as Daddy, may we be filled with Your love and may we, from this love, live those lives that will please you most and lead to all our blessing.
Amen.