To prepare expository sermons, you must be skilled in biblical interpretation or exegesis. In other words, you must be able to determine the meaning of the biblical text and its application to people today. A variety of resources are available to help you grow in your ability to interpret the Bible. (See also, Interpreting the Genres of the Bible.)
Some helpful books on biblical interpretation are the following:
D. A. Carson. Exegetical Fallacies, second edition. (1996)
Moisés Silva. God, Language and Scripture (1991)
Douglas Stuart. Old Testament Exegesis: A Handbook for Students and Pastors, fourth edition (2009) (The first edition can be borrowed here.)
Jason S. DeRouchie. How to Understand and Apply the Old Testament: Twelve Steps from Exegesis to Theology (2017)
Gordon D. Fee. New Testament Exegesis: A Handbook for Students and Pastors, third edition (2002) h(The first edition can be borrowed here.)
Andrew David Naselli. How to Understand and Apply the New Testament: Twelve Steps from Exegesis to Theology (2017)
If you wish to learn more about biblical interpretation, a couple of online courses are also available:
“He Gave Us Scripture: Foundations of Biblical Interpretation,” by Richard L. Pratt, Jr. These eleven video lectures can be found here.
Material from the seminary course, “Biblical Hermeneutics 123” by Vern Poythress is in Powerpoint and pdf formats. These can be found here.