Hi John, why not just tell "your" story. You know, what you were like before becoming a Christian and what you are like now, how life has changed for you because of having Jesus in your life and what He means to you personally.
And if they like what they hear and wish they had what you've found in Christ for themselves, then tell them how you became a Christian (and thereby, how they can become one too

).
Just like a witness is called to testify in a court trial about what they know (not what they don't know), that's what Jesus has principally called us to do as well for the lost in this world. And if they ask you a question that you're not sure of, just be honest and tell them that, that you don't know or you're not sure of the answer to their question or point, but that it's a great question, one that you'd like to know the answer to yourself, and that as soon as you know it, you'll get back to them and let them know too.
Being genuine and honest with people are two of the most important things in witnessing because part of effective witnessing is gaining the trust of the people you're witnessing to. And if being honest means you have to go research something to get them an answer to a question they've asked you, hey, you just set up a second opportunity to share the Gospel with them, didn't you
As you grow in your personal understanding of God, the Bible and the Christian faith, you'll be able to use that new information as well when you witness, but it will never replace your personal testimony about God, what it's like to be a Christian, and how you became one to begin with.
Here is the 4th and final lecture in a series by Dr. John MacArthur called
Spiritual Boot Camp. This lecture is called
How to Witness and it is loaded with helpful tips about doing that effectively (and it's free to listen to and/or read online
here).
God bless you!
Yours and His,
David
p.s. - this is not the normal verse that you'd expect to be reading about witnessing, but it is still one of my very favorite verses in the Bible. When you find the time, take a look at all it says to us about witnessing, about the One we are doing our witnessing for, and who we are to Him as well
Isaiah 43
10 "You are My witnesses,” declares the LORD,
"And My servant whom I have chosen,
In order that you may know and believe Me,
And understand that I am He.
Before Me there was no God formed,
And there will be none after Me".