The First Psalm of the Bible's Old Testament is a wisdom psalm that advises people to be righteous and follow God's law instead of being sinners. The psalm includes some classic Bible symbolism and imagery that compares a righteous person to a tree planted by a stream and a wicked person to wheat chaff blown away by the wind. It is shorter than many of the other psalms.
Psalm 1 serves as an introduction to the rest of the psalms, which number 150 and are divided into categories of praising God, lamentations and wisdom. The First Psalm describes godly people and says their way is to delight in and meditate on the Lord's law. Righteous people are blessed. The psalm states that the Lord watches over the way of the righteous, but the way of the wicked leads to destruction.
The psalm reduces life to two distinct ways that have different results, says Bible.org. If people observe God's law, they will be blessed and prosperous. If people are wicked and don't observe God's law, they will not "stand in the judgment" or be in the assembly of the righteous, presumably after the Last Judgment.
Psalm 1 is a beatitude announcing that God blesses certain people, says Bible.org; but the blessing is conditional. Psalm 1 does not promise that the righteous will have problem-free lives, and one must follow the law to reap the blessing.