Your testimony probably won’t come all at once. It will grow one spiritual experience at a time—as the scriptures say, “line upon line, precept upon precept” (2 Nephi 28:30). You then have to nourish and strengthen your testimony, just like a growing seed (see Alma 32:28–43). And just as a plant can’t survive on water alone, there isn’t just one “best” way to strengthen your testimony. You need a combination of ways.
Prayer and scripture study are definitely good ways to nourish your testimony. As you study the gospel and pray with real intent to know something is true, the Holy Ghost will help you feel the truth of the gospel (see Moroni 10:4–5).
You can also strengthen your testimony by keeping the commandments. When you live a gospel principle—such as fasting, keeping the Sabbath day holy, or taking the sacrament—you start to gain a testimony of how and why it is true, instead of just that it is true. Jesus said that when you choose to “do his will,” you will “know of the doctrine” (John 7:17).
Another way to nourish your testimony is by sharing it. President Boyd K. Packer (1924–2015), President of the Quorum of the Twelve Apostles, said, “A testimony is to be found in the bearing of it!” (“The Candle of the Lord,” Ensign, Jan. 1983, 54; Tambuli, July 1983, 34). You can share your testimony in church and seminary, at home, and with friends. When you do, the Holy Ghost can testify to you that what you are sharing is true.
As you strengthen your testimony in many different ways, it will grow and deepen, and you will feel peace and joy.