There are several things that you as a Christian need to understand about the OT if you are to apply it to your life.
However, God needed to make another covenant with Israel (an additional one for a different purpose) when he brought them into the land hundreds of years later because the Abrahamic Covenant was not given to govern them, but to save them.
This covenant was made with Moses and is called the Mosaic Covenant. The Mosaic Covenant was given to govern Israel as a nation. -40 and 28:1-68 reveal that it was a conditional covenant to the nation.
The New Testament was written to believers in Jesus Christ who had become believers after Jesus died on the cross and the Holy Spirit was given to us at Pentecost. Therefore, many of the exhortations especially in the epistles can be directly applied to us.
The Gospels and Acts are transition books from the OT to the NT times.
This is a covenant that lasts forever and every believer has become a part of that covenant at the moment of salvation.
That covenant has only one provision; you must accept it by faith.
This is also important because the Old Testament is rich in revelation for believers of all ages about the character and nature of God, how he works in the lives of people, and how we can trust and obey Him.
It is also full of revelation that was given directly to Israel and is directly applied to them only.
What we use to interpret the OT in applying it to our lives is the NT.