The book of Joshua is a very positive book in the Bible. It is a book about the faithfulness of God, written to strengthen our faith in his promises, particularly the promise that Jesus Christ is preparing a new home for us where we can enjoy rest in his presence (Hebrews 4:1–16). It is also a book about our responsibility toward God. The Lord has made promises to us. He has shown himself to be worthy of our trust, and in this book we will see how the Lord calls us to now be faithful to him.
In light of the purpose of Joshua, the goal of your sermon must be to show how God has been faithful to us, and what response the Lord is calling us to. For Joshua 24 the character of God in particular is in view and your sermon must draw attention to his power, grace, holiness, and jealousy. In response to who God is, we are called to serve him. Indeed the idea of service appears numerous times in Joshua 24:14–24. Complete devotion (service) is the only reasonable response to who God is.
1 Joshua gathered all the tribes of Israel to Shechem and summoned the elders, the heads, the judges, and the officers of Israel. And they presented themselves before God.