"You cannot teach a man anything; you can only help him find it within himself"
Being a light is nothing more than just living out what you know (Loving and caring for others in spite of how other treat you, training up your children to be responsible, obedient, trustworthy, and caring individuals.) As others see how you live (over a period of time) and they see the outcome of your way of life, then their eyes are opened. They see how they can live to improve their own lives.
For example, today I was out walking our dog Teddy (a Sheltie). As I came to a yard sale I decided to stop and have a look. I heard the owner of the garage sale say, "What a beautiful dog! What kind of dog is that?" I told her that it was a Sheltie, short for Shetland Sheep Dog. She wanted to pet Laddie. After a bit I handing the leash to her so that she could walk Teddie around. She eventually petted Teddie. She had been wanting a Corgie, but after seeing Teddie, she said that she had changed her mind. She called her husband over and told him that she wanted a Sheltie. Before I left I have her my card and the name of the kennel that we got Teddy from.
I realize that this is a very superficial example, but it demonstrates how, when we just live our life, people will be drawn to us if we have something that they want.
Being a light, or witnessing, is not about going around quoting Bible verses to people. Instead, it's living our life in a way that is pleasing to God (being honerable, trustworthy, truthful, considerate, lving, kind, gentle, peaceful), people will take notice. It's nothing that we ourselves do, except to just live our lives.
Jesus said, "....you will be my witnesses in Jerusalem, and in all Judea and Samaria, and to the ends of the earth." Acts 1:7
In a court of law, you can only witness to the things that you personally have seen or experienced. You would not be able to be a witness to something that you have only read about in a book.
John said, "That which was from the beginning, which we have heard, which we have seen with our eyes, which we have looked at and our hands have touched—this we proclaim concerning the Word of life." John 1:1
We are only to witness to the things that we have seen and heard, not that we have merely read in a book. Witnessing is NOT quoting Bible verses at people.