It is the sacred duty of every member of the Church of Jesus Christ of Latter-day Saints to obtain their own personal testimony of their certainty that: the Book of Mormon is true; that Joseph Smith is a prophet of God and that the gospel of Jesus Christ was restored through him; and that the Church is now lead by a living prophet. This certainty is a gift that can only come through the Holy Ghost. It will not come all at once, and it begins with a sincere desire to receive a testimony.
To gain your testimony, if you are 12 or older, read the Book of Mormon and ponder its message. Then ask Heavenly Father to confirm that it is his word. The gift of a testimony from the Holy Ghost usually comes in the form of a quiet assurance—it comes through impressions, feelings, and thoughts, rather than a dramatic sign. For some, this process takes years.
Prophet and church president Thomas S. Monson gained his testimony at age 12, when through hard work he managed to save five dollars. His family didn't own a washing machine, so each week his mother would send their clothes to the local laundry. Young Thomas realized too late that he'd left his $5 bill in the pocket of his jeans. He pleaded with Heavenly Father to keep his money safe until the laundry was returned. When the clothes came back, a wet $5 bill was still in the pocket. Since that time, he has had countless other prayers answered.
Once you have obtained your testimony, it is your obligation to share it with others. Do not wait until your testimony is fully developed to begin sharing it, because part of the development of a testimony comes through the process of telling others about it. Sharing your testimony helps you in your duty to nurture your testimony throughout your life. When sharing with family and friends, your testimony will be most powerful when expressed as a brief, heartfelt conviction rather than a long, wordy discourse.