You witness to someone because you must always be "ready to make a defense to everyone who asks you to give an account for the hope that is in you" (1Pe 3:15). You must make sure you are "accurately handling the word of truth" (2Ti 2:15). You might precede your response with "the Bible says." But the reply might be "how do you know the Bible is true?" It would be a circular argument if you said its true because it says its true. However, you could use logic to substantiate it. Inductive reasoning takes evidence and works its way back to the truth. God accomplished his works to make it known, and one is apparent, where he "brought them up from Egypt" (Isa 8:8). "He saved them for the sake of His name that He might make His power known" (Ps 106:8). If you have the faith to acknowledge that this history is true then, to you, it is a testimony that the Bible is the Word of God.
You should determine what the person's world view is so you can relate to him. Many people believe in evolution and that they are the end product of a very long process proving that their adaptability and intelligence enabled them to prevail. Others are spirit-oriented who believe in other dimensions or realities where the spirit takes precedence over the physical. Your reference must be the Bible and you don't want to go around in circles just talking about philosophies and theories. If you say that the Bible says that God created man you can quote the scriptures where the Bible says it is true. On the sixth day "God created man in His own image" (Ge 1:27) in "spirit and soul and body" (1Th 5:23). "It is I who made the earth and created man upon it" (Isa 45:12). "It was Adam who was first created" (1Ti 2:13). In Hebrew his name means "humanity" and, in an extended sense, the word "flesh" is also humanity. The human race inherits from Adam for, "as in Adam all" (1Co 15:22), applies to everyone. Then "the Lord fashioned into a woman the rib which He had taken from the man" (Ge 2:22) and they "were both naked and were not ashamed" (:25). "God saw all that He had made, and behold, it was very good" (Ge 1:31). The person might be surprised that it sounds logical and consider it.