This is a great question! While I believe you should teach whatever the Lord leads you to teach, if you're looking for suggestions on what books of the Bible to preach first as you begin teaching expositionally, here's a few:

1. Philippians – 4 chapters. Tons of application. Great theology to stretch your hearers.

2. James – Off the charts practical. Great topics to cover that the church always needs to hear.

3. Jonah – Great story. Short book. Not overwhelming.

4. Ruth – Beautiful story. Great opportunity to stretch your hearers with some Old Testament history and great application about God's faithfulness throughout the book

Here's a couple of books that helped me over the years become a better expositor: 

1. Biblical Preaching by Haddon Robinson

2. Rediscovering Expository Preaching by John MacArthur