Answer: It depends, here are a few ideas.
Explanation:
Topic: What will you write about?
Here you should choose what you want to write about. While I don't know if there are any guidelines, some ideas include writing about your new dog, writing a story about a dragon who needs to save a princess dragon from a human, or writing an article regarding how to draw in linear perspective. These are a few of the countless topics that exist to write about.
Purpose: Why will you write?
Here you should provide the reason that you will write about your given topic.
If you chose to write about your new dog, your reason to write may be to express your enthusiasm about your new dog.
If you chose to write about a dragon trying to save a princess dragon from a human, your reason may be to have fun or to explore a new idea.
If you chose to write an article about how to draw in linear perspective, your reason may be to refine your knowledge of the subject or to teach others.
Summary
In conclusion, there are an infinite amount of topics to write about and an infinite amount of purposes to write it.
The topic can range from explaining quantum mechanics to explaining how to draw a smiley face. The purpose can range from trying to have fun to trying to save the world.
Whatever you write, work hard, don't give up, and most importantly, have a great time!