I tried to describe a testimony at it's basics and Elder Ballard really nailed it. But sometimes I wonder how do you gain a testimony. I don't believe anybody wouldn't want such a testimony and witness of Heavenly Father and Jesus Christ. So how does one gain such a testimony?
My dad taught me this simple truth in many different ways. Imagine your testimony is like a puzzle. How do you put a puzzle together? Well obviously piece by piece. Step by step you place the pieces together one at a time to construct an entire picture. Our testimonies are built the same way. You don't gain a testimony all at once, but rather over time and piece by piece.
Those pieces you add to your testimony puzzle come from many different things. Reading the scriptures, you add a piece. You pray sincerely to your Father in Heaven, there's another piece. You pay tithing, there's another piece. You attend sacrament meeting, another piece. The list goes on and on. It's not the acts themselves that provide a piece, it's the Spirit that is felt that confirms the truthfulness of what happened. The the Spiritual things you experience as you keep and live God's commandments and diligently seek Him out are those pieces you add to your testimony puzzle.
For example, my father owned a motorcylce when he was in his youth. He was riding home one day on this motorcycle and was going pretty fast down the road. As he was riding a quite but powerful voice filled his mind and heart. It said "Slow Down". He thought about but really didn't listen to it. He heard it again, "Slow Down". This time he listened to the prompting. As he began to slow the bike's back tire blew. Huge chunks of rubber went flying and one of them caught in the chain. The bike started to fishtail and swerve all over the road. However my dad was able to maintain enough control of the bike and was able to stop on the side of the road. If he hadn't of slowed down, I don't think I would be here.
The Spirit is real and you can experience such promptings. But those experiences don't give you a testimony all at once. This thing that happened to my dad added a huge piece to his testimony puzzle. We can add such pieces to our testimonies if we do the right things. May we strive to do all the rights things so we can add more and more to our testimony puzzle.