Luke 8:16 No man, when he hath lighted a candle, covereth it with a vessel, or putteth it under a bed; but setteth it on a candlestick, that they which enter in may see the light.
Luke 8:17 For nothing is secret, that shall not be made manifest; neither any thing hid, that shall not be known and come abroad.
Mat 10:27 What I tell you in darkness, that speak ye in light: and what ye hear in the ear, that preach ye upon the housetops.
What is this light? It is the truth of God; Jesus is the light of the world. Often Jesus spoke in parables or stories that could be understood as just a story, but for those who saw in the Spirit, there was a spiritual meaning. The new life in Jesus is a new Spiritual life. He gave to us the Holy Spirit of truth, our Comforter and teacher so that we may be able to witness his light of truth in the world.
John 14:26 But the Comforter, which is the Holy Ghost, whom the Father will send in my name, he shall teach you all things, and bring all things to your remembrance, whatsoever I have said unto you.
A new disciple of Jesus may not yet know the deep things of God to share in the world's darkness. Your light may not be as bright as someone with more experience. Do not hide your light under a basket; the light you have is enough to lead another to Jesus. Just tell them how you came to Jesus; perhaps through an affliction or some other experience. Let them know that God sent his son in a human, to pay for sins by dying in our place. Let others know that Jesus has arisen from the grave and was seen by many people to prove that he is the way to everlasting life. This is your personal testimony or witness.
Luke gives a warning to us.
Luke 8:18 Take heed therefore how ye hear: for whosoever hath, to him shall be given; and whosoever hath not, from him shall be taken even that which he seemeth to have.
Be careful what you hear and believe. Is it the truth of God? Or are they the words of a false teacher? We are to hear the word of God, through reading the Bible and through teachers who teach correctly. If we receive error from false teaching, or follow a cult, our light will dim, so be careful what you hear. In the Internet chats there are many false ones, wolves who pretend to be Christians. We grow in faith as we exercise our faith in obedience to the truth of God. What small understanding we may begin with will grow as we serve God. If we receive and believe a lie, then we will not increase in the kingdom God.
John 14:6 Jesus saith unto him, I am the way, the truth, and the life: no man cometh unto the Father, but by me.