Your light must shine before others, that they may see your good deeds and glorify your heavenly Father. ~ Matthew 5:16
Some public speakers say that the structure of a speech should have three parts: tell them what you are going to tell them, tell them; and finally, tell them what you told them. In the Sermon on the Mount, where this shine your light quote comes from, Jesus changes this up a little bit.
In the Sermon on the Mount Jesus tells his disciples
what they are to become (the beatitudes, a blessed disciple, the light of Christian discipleship)
what they are to do with their light (shine their light before others)
and finally the things necessary to become a blessed disciple (love enemies, forgive, etc.).
Periodically read the sermon on the Mount found in Matthew chapters 5, 6 and 7. This is Jesus, the son of God, the image of the invisible God, telling us how to model our lives. Some of the teachings are difficult, such as loving your enemy, but God would not ask us to do the impossible. With God all things are possible.