Being a witness for Jesus is not something that we say or do; being a witness is something that we are. In Acts 4: 13 is the perfect description. The rulers, elders, and scribes reacted in this way to the witnessing of Peter and John:
"Now as they observed the confidence of Peter and John, and understood that they were uneducated and untrained men, they were marveling, and began to recognize that they had been with Jesus."
If you, too, spend time with Jesus, learning of Him, becoming more and more like Him, you will be His witness. You cannot help yourself.
"Now he who plants and he who waters are one; but each will receive his own reward according to his own labor. For we are God's fellow-workers... So then neither the one who plants nor the one who waters is anything, but God who causes the growth."
1 Corinthians 3: 8, 9, 7
Jesus said, "Every plant which My heavenly Father did not plant shall be rooted up."
Matthew 15: 13
"Nowadays the great passion is the passion for souls... It is the passion for Christ that the New Testament teaches. It is not a passion for men that saves men. Whenever the passion for souls obscures the passion for Christ, Satan has come in as an angel of light."