Dreams are influenced by several things. One of the ways they are influenced is by daily life, and how your subconscious chooses to project your emotions/feelings in the dream. For example, if you are insecure about your looks; you may have a dream about being unclothed in a public place. Another way you can effect your dreams (as another commenter has said) Is to focus on what you want to dream about until you are unconscious.
It is best to focus on this thought until you fall asleep, instead of normally letting your mind wander.