Why do we have introductions? They are there to attract readers, tell them roughly what to expect, and give them a clue to your tone and purpose. Introductions also help readers to assess their knowledge and views about your topic.
It does not matter whether you write your introduction first as a way of orienting yourself or if you do it later when you are sure about what your essay contains.
There are many different ways you can choose to open your piece of writing. This will in part depend on what you are writing: a discussion, a debate, a story, a news item and so on.
You can start with a question. It certainly starts the reader thinking about your topic. Make sure, though, that the question is not too obvious.
You can start by telling your readers what to expect. You mention the topic and explain how you are going to treat it. This gives the essay a very clear structure and can be very helpful with a difficult subject.
You can start with a description of a situation that leads you into the topic. An essay on divorce could start with a family argument, a distressed child or a court scene depending on your purpose.