You can write a book if you will give a detailed answer to this question.
Let’s try to look at some verses that gives us light about the main differences between the old and new testament.
First, the New Testament is the testament that is in authority right now.
Long ago, at many times and in many ways, God spoke to our fathers by the prophets,
but in these last days he has spoken to us by his Son, whom he appointed the heir of all things, through whom also he created the world.
Before, God speaks to us through the prophets such as Moses and Elijah (the Old Testament) but now God speaks to us through His son and Christ’s commandments are in the New Testament.
This is what God said and emphasized at the mount of transfiguration.
4Peter said to Jesus, “Lord, it is good for us to be here. If You wish, I will put up three shelters—one for You, one for Moses, and one for Elijah.” 5While Peter was still speaking, a bright cloud enveloped them, and a voice from the cloud said, “This is My beloved Son, in whom I am well pleased. Listen to Him!”
As we can see here, God didn’t allow Jesus to be equal with Moses(the law) and Elijah (prophets), that is why he interrupted Peter and stated that we have to listen to Jesus now.
Does this mean that the Old Testament is useless. Far from it.
Romans 15:4 King James Version (KJV)
4 For whatsoever things were written aforetime were written for our learning, that we through patience and comfort of the scriptures might have hope.
@zayn22 There are so many differences I will just highlight but a few.
There is a difference in time period. The Old Testament books were written from the time of Moses until about 400 BC. This approximately 1,000-year period covers a wide range of events from the creation of the universe to the return of the Jewish people to Jerusalem under Ezra and Nehemiah. The New Testament was written between approximately AD 50 and AD 150.
There is difference in focus. The Old Testament chronicles God’s power and works, looking toward the coming Messiah. The New Testament reveals Jesus as the Messiah, looking back at His life and teachings as the basis for the church and the spread of the gospel.
There is a difference in prophecy. Most Old Testament prophecies look forward to the future and were unfulfilled at the time of the Old Testament’s completion. The New Testament highlights the fulfillment of many of the Old Testament’s prophecies, including more than 300 in the life of Jesus Christ.
There is a difference in the practice of worship. Much of the Old Testament focused on the tabernacle or temple as the central place of worship. Many details were included in the development of offerings, festivals, and related practices. In the New Testament, Jesus offered Himself as the focus of worship, claiming to be the way, the truth, and the life, and that no one could come to the Father except through Him (John 14:6).