(15) See then that ye walk circumspectly, not as fools, but as wise,
(16) Redeeming the time, because the days are evil.
(17) Wherefore be ye not unwise, but understanding what the will of the Lord is.
(18) And be not drunk with wine, wherein is excess; but be filled with the Spirit;
(19) Speaking to yourselves in psalms and hymns and spiritual songs, singing and making melody in your heart to the Lord;
(20) Giving thanks always for all things unto God and the Father in the name of our Lord Jesus Christ;
"Speaking to yourselves in psalms and hymns and spiritual songs" is related to "be filled with the Spirit." The parallel passage is Colossians 3:16-17, "Let the word of Christ dwell in you richly in all wisdom; teaching and admonishing one another in psalms and hymns and spiritual songs, singing with grace in your hearts to the Lord. And whatsoever ye do in word or deed, do all in the name of the Lord Jesus, giving thanks to God and the Father by him."
We are not only to speak to ourselves in psalms, hymns, and spiritual songs, but also to use them to teach and admonish one another. This hints that the songs we sing should be grounded in the Word of God - for "All scripture is given by inspiration of God, and is profitable for doctrine, for reproof, for correction, for instruction in righteousness: That the man of God may be perfect, throughly furnished unto all good works." (2 Timothy 3:16-17) If a song is not doctrinally sound, should it be on our lips? Should we use it to sing to others?
The songs are to be in our heart - unto the Lord. What song is in our heart? The songs of the world? or a "new song?"
"Sing unto him a new song; play skilfully with a loud noise." (Psalm 33:3)
"And he hath put a new song in my mouth, even praise unto our God: many shall see it, and fear, and shall trust in the Lord." (Psalms 40:3)
"O sing unto the Lord a new song: sing unto the Lord, all the earth." (Psalms 96:1)
"O sing unto the Lord a new song; for he hath done marvellous things: his right hand, and his holy arm, hath gotten him the victory." (Psalms 98:1)
"I will sing a new song unto thee, O God: upon a psaltery and an instrument of ten strings will I sing praises unto thee." (Psalms 144:9)
"Praise ye the Lord. Sing unto the Lord a new song, and his praise in the congregation of saints." (Psalms 149:1)
Besides singing, we are to give thanks unto God. For all things. In the name of Jesus Christ. 1 Thessalonians 5:18 makes it clear that this isn't just a suggestion, but a command, "In every thing give thanks: for this is the will of God in Christ Jesus concerning you."