Q: What do the scriptures principally teach?
A: The scriptures principally teach what man is to believe concerning God, and what duty God requires of man.1
1. Micah 6:8. He hath shewed thee, O man, what is good; and what doth the Lord require of thee, but to do justly, and to love mercy, and to walk humbly with thy God?
John 20:31. But these are written, that ye might believe that Jesus is the Christ, the Son of God; and that believing ye might have life through his name.
John 3:16. For God so loved the world, that he gave his only begotten Son, that whosoever believeth in him should not perish, but have everlasting life.
When we approach the Scripture to find out what job I should take or if I should by this business or not, we are seeking from the Scripture something that is askew from the main purpose. That is not to say there is no good in seeking guidance and wisdom, which often does come from Scripture. If I wonder if I should explore serving other God's besides the one true living God, the Scripture does tell me what to do. God of the Bible forbids it. While my country does not specify whom I should serve in religion, the Bible tells me whom I am to serve.
In deciding what to read in the Bible there are two approaches to take. The first is to figure out what issues in our life need addressing then read the book of the Bible or select passages that deal with that passage. Often our questions that we would like the Bible to answer do not line up with that 3rd Question and Answer of the Westminster Shorter Catechism. Our primary questions should be in line with our primary needs, but sometimes our hearts are not ready for that. Our hearts ask for questions about suffering, marriage, finances, living a good life, and how to be wise. The Scripture does address these, so going to those passages addressing those sorts of questions that are on our hearts can be an entrance to Scripture reading. However, if we see the Bible as just a self-help book helping us with temporal concerns in the here and now, we may never get to the greater teachings of Scripture. We must eventually look at deeper questions such as who is God? Who is Jesus? What is salvation? Salvation from what? What is sin? How do I serve God?
What do I read next based on my need? You can choose a temporal, earthly need or an eternal issue. Either one can be a starting point. It should not be an end though.
One way to address the topical issues is to look at Nave's Topical Bible. Billy Graham in his ministry recommended this tool and there are versions on line.
Another way to find topical Scripture passages is to use Concordances. Today this activity can be easily done by searching for words in Bible Software. There are many ways this can be accomplished. There are Bible apps and applications. Some of them for free such as put out by http://CrossWire.org. There are websites that have the whole Bible on them as searchable text. Examples: Biblegateway.org, netbible.org, and esv.org.
Google can also be helpful by Googling phrases like, "What the Bible says about wisdom". The auto complete of "What the Bible says about" can tell you a bit of what is on people's minds.
The other way to read the Bible is to read through the whole thing. Ligonier Ministry has a bunch of different Bible reading plans on a page dedicated for this purpose. There are charts for just reading one chapter at a time over three years. There Bible reading plans that take one through the whole Bible in a year. There is the M’Cheyne plan which takes one through the whole Bible in a year, but takes one through the Psalms and New Testament a second time. The ESV.ORG site also has reading plans.
For myself, I am reading shorter passages but listening to a podcast which reads the ESV with the M'Cheyne plan.