I found the following paragraph from Louis Berkhof (1873-1957) helpful as I come to greater grips with how to read (and preach) the Bible. I hope you find it helpful too:

The Churches of the Reformation from the very beginning distinguished between the law and the gospel as the two parts of the Word of God. This distinction was not understood to be identical with that between the Old and the New Testament, but was regarded as a distinction that applies to both Testaments. There is law and gospel in the Old Testament, and there is law and gospel in the New. The law comprises everything in Scripture which is a revelation of God's will in the form of command or prohibition, while the gospel embraces everything, whether it be in the Old Testament or in the New, that pertains to the work of reconciliation and that proclaims the seeking and redeeming love of God in Christ Jesus.

Systematic Theology (Grand Rapids, 4th ed. 1941, pg. 612)