Luke 13

Then He said, “What is the kingdom of God like? And to what shall I compare it?  It is like a mustard seed, which a man took and put in his garden; and it grew and became a large tree, and the birds of the air nested in its branches.” vv.18-19

Read chapter 13
What is the Kingdom of God?  As we saw in the last chapter, chapter 12, we are to seek first His Kingdom.  How are we suppose to seek something we do not know what we're looking for?  Study of God's scripture!  The Kingdom of God is not just church or christianity as some refer it to, remember the word kingdom means government, the government of God is what we are to seek.
Here the Lord uses a comparison to help his hearers understand what the kingdom of God is like.  Bible teacher Nathan Johnson explains this verse  well: 

The Lord compares the kingdom of God to a mustard seed which a man plants in his garden. This is a very unusual illustration from the start, for who in planting a garden would ever plant just a single seed? Would a farmer planting corn ever plant just a single seed of corn? Would one planting wheat plant a single grain of wheat in the ground? This is not how planting is done! Yet this man plants only a single mustard seed. He must have great confidence indeed in this seed to do such a thing!
Mustard seeds were very small, yet they grew into a rather large plant. Yet this plant was pretty much a bush, and in no wise could be called a large tree. Yet this mustard seed in Christ’s illustration grows beyond all expectation for such a seed. It grows into a great tree, in which the birds of the air can come and nest in its branches.
Thus according to the Lord, the Kingdom of God starts very small. Yet ultimately it will grow so large that those who see it will be astonished, and declare that no one would have expected it to grow so large considering its humble beginnings and considering how much larger it has become than all other governments in the past have ever been. As we look at the record of God’s government in the New Testament, we can see with what small beginnings the Lord began His movement in the earth. Whether we count the beginning as a single babe in a manger in Bethlehem, or as a little group of 120 men gathered together on the day of Pentecost, the kingdom began in a very small way indeed. Yet what a large and all-encompassing government will grow out of these very small beginnings!

You may not plan seeds so this illustration isn't as clear to you, but what can you relate a mustard seed like to you in comparing this point of what the kingdom of God is like?