"He (Jesus) also said, 'The kingdom of God is as if someone would scatter seed on the ground, and would sleep and rise night and day, and the seed would sprout and grow, he does not know how. The earth produces of itself, first the stalk, then the head, then the full grain in the head. But when the grain is ripe, at once he goes in with his sickle, because the harvest has come.'" (Mark 4:26-29)
Time and again in the Gospels, Jesus uses imagery to explain to his disciples, and to us, how we are to live in God's new kingdom. Part of our "job" as citizens of this kingdom is to share God's love and grace with others. We can and should, of course, share our faith with others by telling them of Jesus; however, that doesn't complete the picture. We are told to be sowers of seed, fishers of men, a light on a hill, salt that hasn't lost its flavor, and so on.
This parable reminds us that we are to plant seeds in any and every way possible - by sharing the story of our faith, by letting others see our love for God and for them, by living in such a way that people begin to wonder what makes us different - and then not worrying about the outcome. Just like the earth will grow a seed, God will take your and my tiny contribution to the kingdom and grow it into something bigger. But for that to happen, seeds need to be planted.
Every time you speak of your faith, every time you invite someone to church, every time you show God's love to someone through something you do, think of it as planting a seed. Some seeds don't grow, others begin to grow and then fail to thrive for some reason, and then there are others that grow into plants that are harvested and produce their own seeds. Visualize that you have, in your possession, a large sack of seeds given to you by God; now visualize yourself planting those seeds, one at a time, and praying that they take root and grow.