Which testament are you supposed to read first, the New Testament or the Old Testament?
Nothing requires that you read either first. Some reading plans include parts of each testament in each day’s reading. Parts of the New Testament books of Romans and Hebrews, as well as parts of the life of Christ are difficult to understand without at least a passing familiarity with the Old Testament.
I usually recommend someone start with the life of Jesus, such as the book of John or Luke, move on to Acts, and then read some of the other books (especially the historical books of the Old Testament). After reading the historical books, I recommend the letters of Paul and other apostles, the Psalms and Proverbs. As a new reader of the Bible, I would save the books of prophecy for last.