Have you ever read a ‘whodunnit’ murder mystery novel? Did you read the last chapter first? Why not? I’m guessing it was because it wouldn’t have made sense without the preceding chapters which gave context to the final chapter and made the final chapter understandable.
In a similar way, the New Testament is the final chapter of the Old Testament. It is the fulfillment of the Old Testament. The Old and New Testaments work together to give us the whole picture of who God is, who we are and what God does with our rebellion. The Old Testament forms a backdrop against which we can understand the New Testament. The New Testament is the fulfillment of God’s plans, purposes and promises which he gave in the Old Testament. As such then you can say that both the Old and New Testaments are about Jesus - the Old Testament points to Jesus, and the New Testament describes his life, death and resurection, and reflects on the impact this has in the world.
2 Corinthians 1:20 says, “For no matter how many promises God has made, they are “Yes” in Christ.” Jesus says “Do not think that I have come to abolish the Law or the Prophets; I have not come to abolish them but to fulfill them. I tell you the truth, until heaven and earth disappear, not then smallest letter, not the least stroke of a pen, will by any means disappear from the Law (1st 5 books of the Bible) until everything is accomplished.” (Matt 5:17-18).
Without the Old Testament we would not understand as well the original problem to which Jesus is the solution, that being our sin and rebellion against God (Genesis 3); we would miss out on reading about God’s mighty acts of rescue through which we see his character on display (eg in the book of Exodus). we would miss out on the predictions of Jesus death and resurrection which were 1000 years before they occurred exactly in Jesus. In short we would not understand Jesus as well as we do without the Old Testament. As we read and reread the Old Testament, what Jesus has accomplished on our behalf will become clearer and clearer still.
If you wish to find out more I would highly recommend a book called “Gospel and Kingdom” by Graeme Goldsworthy which is available (as part of a trilogy) at Amazon