Core Facts of the Gospel

"For I delivered to you as of first importance what I also received: Christ died for our sins in accordance with the Scriptures, that he was buried, that he was raised on the third day in accordance with the Scriptures."
1 Corinthians 15:3-4

WHAT MUST ONE DO WITH THE GOSPEL TO BE SAVED?

Recognize that you are born a sinner and need forgiveness for your sins by confessing them to the LORD. (1 John 1:9)
Believe the core facts of the Gospel. (John 316)
Receive the Lord Jesus Christ into your life to be your Lord and Savior. (John 1:12)

How to Write Your Personal Testimony.

Introduction
Show your need of a Savior by briefly explaining what your life looked like before Christ. What were your relationships like before finding Christ? How were not living according to God's Word?
How you placed your faith in Christ
Go on to explain what made you come to the point of surrendering your life to Christ. You can even explain what the Gospel is here so that another may be able to understand and come to Christ. Use your testimony to point people to God. 
Your life after coming to Christ
Explain a couple of ways in which your life has changed since surrendering to Christ. These can tie into the examples in your introduction of you falling short. It is important show changes in who you are and not just your behavior. Also, it might be good to emphasize that struggles and sin does not stop after salvation, it is a constant process to be sanctified.
Closing
Think of a brief closing/summary statement to wrap up your testimony. It does not have to be anything fancy but try to make it all connect together.