Read widely on topics that pertain to interest and passion. Since interest and passion are broad ideas you are going to need to narrow the essay into a particular focus. One way to do so would be to find examples in history where interest and passion influenced people into changing events. Another way would be to read psychology publications about interest and passion's effects on the brain. The key is to choose an angle and stick with it.
Explain a theory based on your reading. Focus on something that you can back up with your explanations, connections and examples. "Interest and passion are good things" is not a very good argument. "Interest and passion have influenced many key historical events," is more plausible since it can be backed up with facts and examples.
Jot down key points that back up your thesis statement. The more points you can find, the longer your essay will be, since each point will turn into a paragraph.
Write an outline. Each outline's heading will turn into your topic sentence for your paragraphs, while each subheading will turn into a supporting sentence. Each section of the outline should make a separate point that backs up your argument and each subheading should explain or exemplify it.
Turn your outline into a rough draft by writing the bullet points as sentences.
Edit for grammar, style and clarity, make any needed rewrites and turn in your essay.