"Mortify the deeds of the body" in verse thirteen means to put to death or regard as dead the power of sin in your body.
The apostle Paul uses adoption to illustrate the believer's new relationship with God. In Roman culture, the adopted person lost all rights in his old family & gained all the rights of a legitimate child in his new family. He became a full heir to his new father's estate. Likewise, when a person becomes a Christian, he gains all the privileges & responsibilities of a child in God's family. One of these outstanding privileges is being led by the Spirit. We may not always feel like we belong to God, but the Holy Spirit is our witness. His inward presence reminds us of who we are & encourages us with his love.
Because we are God's children, we share in great treasures as joint-heirs. God has already given us his best gifts: his Son, forgiveness, & eternal life; & he encourages us to ask him for whatever we need.
There is a price for being identified with Jesus. Along with the great treasures, the apostle Paul mentions the suffering that Christians must face. In many parts of today's world, Christians face pressure just as severe as those faced by Christ's first followers. Even in countries where Christianity is tolerated or encouraged Christians must not become complacent. Nothing we suffer however, can compare to the great price Jesus paid to save us.
Reference summary used from the Life Application Bible KJV, Tyndale House Publishers, Inc., Wheaton, Illinois