3 Nephi 13: 22-23
"The light of the body is the eye; if, therefore, thine eye be single, thy whole body shall be full of light. But if thine eye be evil, thy whole body shall be full of darkness. If, therefore, the light that is in thee be darkness, how great is that darkness!"
The "eye" is better put "your mind's eye." It is what you meditate on, what fills you. You choose what you fill yourself with by what you give attention. What you notice is what you care to notice.
Christ's admonition is troubling because the cares of this world distract us all. They impose upon us all. But Christ advises us to search endlessly for light.
The difference between filling yourself with light and filling yourself with darkness is what thoughts you entertain.
Everything begins in the mind. Words and works flow from thoughts. (Alma 12: 14.) While all three will be judged, it is in the mind where all else begins.
It is not enough to attempt to avoid evil by memorizing hymns. You can spend as many wasted hours humming hymns as singing rock songs. Neither one will particularly elevate you. Meditating on doctrine, pressing understanding, pondering deeply and engaging the mysteries of God are what will fill the mind with light.
There is so much in our faith that distracts and substitutes for light and truth. Think about these verses and filling your mind with light and truth: "And that which doth not edify is not of God, and is darkness. That which is of God is light; and he that receiveth light, and continueth in God, receiveth more light; and that lightgroweth brighter and brighter until the perfect day. And again, verily I say unto you, and I say it that you may know the truth, that you may chase darkness from among you;" (D&C 50: 23-25.)