Psalm 1 begins with a promise.
The opening psalm teaches that blessedness comes with meditating on God's Word. The Psalmist writes that a blessed person takes "delight in the law of of the Lord" and "meditates day and night" on God's Word.
Daily meditation on the promises of God in Scripture provides assurance of God's providence and grace - in the world and in your life. Reading and reflecting on God's Word leads to peace and contentment because it turns your mind and heart to the Triune God.
Alternatively, if you do not meditate on God's Word you will be inclined to follow those who reject the Lord ("the counsel of the wicked" in v. 1).
The Psalmist reminds you of a daily choice - to place your thoughts upon the promises of the eternal God or to take counsel from the foolish who live for themselves.
If you meditate and live according to God's Word you will be "like a tree planted by streams of water that yields fruit in its season, and its leaf does not wither." In other words, spiritual reflection on biblical truth leads to spiritual flourishing and spiritual life. However, following the example of the godless leaves you "like chaff that the wind drives away."
Psalm 1 begins with a promise. It also ends with a promise.
"The LORD knows the way of the righteous." God the Father cares for those who belong to him and he holds them in his hands.
The Father cares for his children because his Son, the Lord Jesus Christ, died on the cross to redeem them from sin, death, and the devil. This psalm reminds you to daily meditate on the promises of God and live today in light of the redemptive death and resurrection of the Son of God.