EDIT: You may be looking for the more thorough explanation of what my testimony is and how I got it that I wrote in August 2012.
I know that Jesus Christ is the Son of God.
This message has always been carried forth by the Holy Ghost. "Angels speak by the power of the Holy Ghost; wherefore, they speak the words of Christ" (2 Nephi 32:3). The prophet and missionary Ammon said, "A portion of that Spirit dwelleth in me, which giveth me knowledge, and also power according to my faith and desires which are in God" (Alma 18:35). The Holy Ghost witnesses of Jesus Christ (2 Nephi 31:18).
The Apostle Paul taught that know man can truly know that Jesus Christ is the Messiah any other way. He said, "No man can say that Jesus is the Lord, but by the Holy Ghost" (1 Corinthians 12:3). How can anyone say that Jesus is the Lord with complete surety without the Holy Ghost? Seeing the Lord is a rare privelege that we cannot demand. But we can ask the Lord for the privelege of knowing, for ourselves, that Jesus Christ is the Son of God.
I have asked the Lord for the privilege of having this testimony. For me, it started out by asking if the Book of Mormon was true (Moroni 10:3-7). The Book of Mormon states that Jesus is the Christ. It also teaches many other gospel principles, and so I also know that those are true. The Book of Mormon was translated by Joseph Smith, and so I know that he was a prophet of God and that he saw the Savior with his own eyes.
Most of all, I know that Jesus Christ is the Son of God.