Remember that you are not writing a biography. Your testimony should build a bridge to the people you are talking with, explaining why and how you became a Christian, and the difference Jesus has made in your life.
The people listening should have enough information to become a Christian if they want to.
Be brief -- less than 3 minutes (650 words). You can expand later.
Build on common ground in your introduction. What can you and your audience agree upon?
Use concrete terms. Your listeners should be able to taste, touch and feel your conversion.
Answer the following questions:
What were you like before you became a Christian?
How did you come to know Christ?
What was your life like after you received Christ?
How would you describe your relationship with God now?
No more than 1/3 of your talk should be about life before you were a Christian.
If you became a Christian as a small child, concentrate on your relationship with God now and also share why you have maintained your commitment to Christ as an adult.
A Bible verse may be helpful, but don't give the complete reference. Non-Christians don't know what "Jude" is anyway. Simply say "it says in the Bible that. . ."
Be positive, not negative, but realistic, from start to finish.
Ask the Lord to give you wisdom and guidance as you write (James 1:5,6).
Re-write your testimony until it communicates. Many people need to write 3 or more drafts.
Making negative statements about the church, other Christian organisations or people.
Preaching at people. You are sharing your life story, not giving a sermon.
Using words that are hard to understand. In particular avoid Christian words that do not make sense to those outside the Christian culture (even words like "sin" and "received Christ" need to be defined).
Knowing God in a personal way
The realization that I am sinful and unacceptable to God - even though I do religious things
Assurance of salvation and eternal life