Don't be discouraged; Jesus said that even if our faith is only as big as a mustard seed (which is very small), God can still bless it and use it and cause it to grow (see Luke 17:6).
How can your faith grow? Think for a moment about a seed and how it grows. As long as it's in the package or sitting on a shelf, nothing will happen; it will still be a tiny seed. But once we make a decision to plant it, and then actually place in the soil and water it properly, it will sprout and begin to grow, and eventually produce fruit.
The same is true with our faith. It begins to grow with a decision -- a decision on our part to commit our lives to Jesus Christ and trust Him as our Lord and Savior. Then we nurture our faith by surrounding it with the "soil" God has given us -- the "soil" of the Bible, prayer and fellowship with other believers. In one of His parables, Jesus compared the Gospel to seed that a farmer sows in his field. When we hear God's Word and understand it and apply it to our lives, He said, it "produces a crop, yielding a hundred, sixty or thirty times what was sown" (Matthew 13:23).
Begin by asking Jesus Christ to come into your life. He loves you; He loves you so much He gave His life for you. Then surround yourself with the things God has given us to strengthen our faith: the Bible, prayer and fellowship with other believers.