The purpose of this psalm is threefold: 1) the worshippers declare their faith to one another, to any who will listen, and to the very angels of God; 2) they pray the prayer of faith; 3) they encourage one another in faith.
The first six verses of the song are the declaration of faith. The people sing, "The Lord is my light and my salvation; whom shall I fear. The Lord is the strength of my life; of whom shall I be afraid?"
They're just saying that God is the One Who can shed light on problems, difficulties, and even the meaning of life. Relationship to Him, in fact, is what makes life meaningful, supplies overcoming grace, and inspires the fortitude needed in every situation. No matter who or what the problem, GOD is the answer. They sing, "Though an army may encamp against me, my heart shall not fear; though war may rise against me, in this I will be confident!"
Having such complete confidence, there's only one thing more needed. They sing next, "One thing I have desired of the Lord, that will I seek - that I may dwell in the house of the Lord all the days of my life, to behold the beauty of the Lord, and to inquire in His temple." The idea is that it's great to be in the temple singing this song, but the real need is to stay there, to always be inquiring there - never to go elsewhere for help. If God grants this prayer, then everything will indeed be all right: "He shall hide me; He shall set me high upon a rock."
Having prayed, and, knowing the grace of the prayer-hearing God - that He WILL indeed allow the worshipper always to thus dwell in the house of the Lord - he should then be confident. He can then sing, "And now my head shall be lifted up above my enemies all around me; therefore I will offer sacrifices of joy in His tabernacle; I will sing, yes, I will sing praises to the Lord!"
Upon the foundation of such faith, then, each worshipper can now pray the prayer of faith. He can joyfully sing, "Hear, O Lord, when I cry with my voice! Have mercy upon me, and answer me!" He knows God Himself has called him to such prayer. He sings, "When You said, 'Seek My face,' my heart said to You, 'Your face, Lord, I will seek.'" He knows that, even in the most unimaginably extreme emergency, even if his own father and mother forsake him, then - even then - "the Lord will take care of me!"
With such confidence, the worshipper can pray on: "Teach me Your way, O Lord, and lead me in a smooth path, because of my enemies." He sings that he would have lost heart if he hadn't believed that he would indeed "see the goodness of the Lord in the land of the living."
Having such solid, joyful faith in the Lord, the worshippers now turn to each other and sing, "Wait on the Lord; be of good courage, and He shall strengthen your heart; wait, I say, on the Lord!"
Do you see God as your light in life - is your relationship with Him the foundation for everything you do? Is He the One you're seeking today? Do you love Him, so that your ultimate aim is to go and be with Him forever? Based on that relationship, do you pray as He's invited you to pray? And do you share that joy as often as possible with brothers and sisters in the congregation of those who love and worship Him? That's the way it should be in the life of every Christian!