Doubt is essentially a lack of faith.

How does one get faith?

So then faith cometh by hearing, and hearing by the word of God.
- Romans 10:17 (KJV)

It's basically a day-to-day commitment to the Word of God. Studying the Bible, and praying.

The Bible reveals the character of God, and one of the biggest things to understand about God is that He will always keep His Promises.

Man wavers every now and then, and in fact, this happens to the best of us.

And straightway Jesus constrained his disciples to get into a ship, and to go before him unto the other side, while he sent the multitudes away. And when he had sent the multitudes away, he went up into a mountain apart to pray: and when the evening was come, he was there alone. But the ship was now in the midst of the sea, tossed with waves: for the wind was contrary. And in the fourth watch of the night Jesus went unto them, walking on the sea. And when the disciples saw him walking on the sea, they were troubled, saying, It is a spirit; and they cried out for fear. But straightway Jesus spake unto them, saying, Be of good cheer; it is I; be not afraid. And Peter answered him and said, Lord, if it be thou, bid me come unto thee on the water. And he said, Come. And when Peter was come down out of the ship, he walked on the water, to go to Jesus. But when he saw the wind boisterous, he was afraid; and beginning to sink, he cried, saying, Lord, save me. And immediately Jesus stretched forth his hand, and caught him, and said unto him, O thou of little faith, wherefore didst thou doubt? And when they were come into the ship, the wind ceased. Then they that were in the ship came and worshipped him, saying, Of a truth thou art the Son of God.
- Matthew 14:22-33 (KJV)

We must learn to trust God fully, and surrender to Him every day.

The best way to learn that is to study God's Word.