Summary: the psalmist explains to us What we do, what we know, and why be Thankful to GOD:
In this fourth book or collection of the Psalms 90 (quickview)  – 106 whose authorship has been ascribed to the hands of Moses. In this writing the Psalmist, in an special manner, invites believers to praise God, because he has chosen them to be his people, and has taken them under his care. Just hearing or reviewing the words.. it comes to mind how through their deliverance from the harshness of labor, the impending death from the death angel, and deliverance from the Egyptian captivity.
With this in mind; the psalmist explains to What we do, what we know, and why be Thankful to GOD:
I. What should we do?
Serve Him - If any man serve me, let him follow me; and where I am, there shall also my servant be: if any man serve me, him will my Father honour. (John 12:26 (quickview) ) (KJV)................................................................................
a. Make a joyful noise unto the Lord all ye lands
b. Serve the Lord with gladness
c. Come before His presence with Singing
II. What should we know?
He alone is God - "That men may know that thou, whose name alone is JEHOVAH, art the most high over all the earth."
a. Know ye that the Lord HE IS GOD
b. It is He that have saved us
c. Not we ourselves
III. Why should we do it?
His Truth Endures FOREVER - Before the mountains were brought forth, or ever thou hadst formed the earth and the world, even from everlasting to everlasting, thou art God. (Psalm 90:2 (quickview) )
a. the Lord is good.
b. His mercy is everlasting
c. His truth endureth to all generations
In conclusion, make a joyful shout to the LORD, all you lands! Serve the LORD with gladness; Come before His presence with singing. Know that the LORD, He is God; It is He who has made us, and not we ourselves;We are His people and the sheep of His pasture. Enter into His gates with thanksgiving, And into His courts with praise. Be thankful to Him, and bless His name. For the LORD is good; His mercy is everlasting, And His truth endures to all generations