Q: What is required in the first commandment?
A: The first commandment requireth us to know and acknowledge God to be the only true God, and our God, and to worship and glorify him accordingly.

The exposition of the Ten Commandments found in answers to Questions 46-81 are deductions from the commandments themselves and the rules set forth in the Larger Catechism, Q. 99. The texts under the specifications are given to show that they are in accord with the general teaching of the Scriptures.
1 Chronicles 28:9. And thou, Solomon my son, know thou the God of thy father, and serve him with a perfect heart and with a willing mind: for the LORD searcheth all hearts, and understandeth all the imaginations of the thoughts: if thou seek him, he will be found of thee; but if thou forsake him, he will cast thee off for ever.
Deuteronomy 26:17. Thou hast avouched the LORD this day to be thy God, and to walk in his ways, and to keep his statutes, and his commandments, and his judgments, and to hearken unto his voice.
Matthew 4:10. Then saith Jesus unto him, Get thee hence, Satan: for it is written, Thou shalt worship the Lord thy God, and him only shalt thou serve.
Psalm 95:6-7. O come, let us worship and bow down: let us kneel before the LORD our maker. For he is our God; and we are the people of his pasture, and the sheep of his hand. Psalm 29:2. Give unto the LORD the glory due unto his name; worship the LORD in the beauty of holiness.

Westminster Project - See John Flavel on the First Commandment