The master thought of this psalm is the law of The LORD. The obedient and disobedient are placed in sharp contrast. This contrast is vividly seen by bringing together the first and last words of the psalm: "blessed," vs "perish." The former word describes the issue of obedience; the latter, the result of disobedience.
The conditions of blessedness are stated negatively and positively. Negatively, there must be complete separation from fellowship with those who are disobedient. The graduation in description must not be omitted; "walketh," "standeth," "sitteth"; "counsel," "way," "seat"; "wicked," "sinners," ''scornful." The positive condition is twofold delight and meditation in the Law. Moreover, this must be continuous, "day and night."
The Way of the Righteous and the Wicked      
Psalms 1:1
Blessed is the man that walketh not in the counsel of the ungodly, nor standeth in the way of sinners, nor sitteth in the seat of the scornful.  QUIZ 
 
Psalms 1:2
But his delight is in the law of the LORD; and in his law doth he meditate day and night.        Spurgeon    
Psalms 1:3
And he shall be like a tree planted by the rivers of water, that bringeth forth his fruit in his season; his leaf also shall not wither; and whatsoever he doeth shall prosper.
Psalms 1:4
The ungodly are not so: but are like the chaff which the wind driveth away.
Psalms 1:5
Therefore the ungodly shall not stand in the judgment, nor sinners in the congregation of the righteous.
Psalms 1:6
For the LORD knoweth the way of the righteous: but the way of the ungodly shall perish.