The most important thing to do is to review the specifications that your teacher has laid out for the assignment. If you teacher just said "write a response", then I think you'd be justified in asking for a further explanation of what they mean and what they expect. However, instructors usually provide something like this when the assignment is given, and I would imagine yours is no different. Make a bullet list of the requirements and build your response around them, or try making a "map" of your essay that lists the requirements and connects them to the ways you're going to fulfill them.
In my experience, when instructors assign response-style essays, they're looking for a few specific things;
- Evidence that you actually read the story, i.e. quotes and multiple references within the text
- Your ability to to express your opinions and tie them to specific influences
- An emotional investment
The last part is usually the hardest; I've had many students argue that they felt nothing, they didn't care about the story, etc. If this is the case, reflect upon why you don't care or aren't interested: what about the story and characters was not engaging, referring to specific instances or descriptions that didn't catch your interest. Do not reflect upon what you would do differently or what the story did wrong; only upon yourself and your reactions to specific things within the text.
Begin your essay by asserting your feelings, and then connecting them to the story. Finish the first paragraph with a thesis-style statement, such as "I felt that The Wednesday Wars used religion as an unnecessary and inflammatory means to an end, one which is more likely to alienate opinionated readers than to engage them in the emotional frame of the main characters." In the following paragraphs, articulate upon this statement by drawing on specific examples in the text, examples from your own life which support your position, and include or counter contradictory arguments. Conclude by articulating your original statement in a more general way; "Authors must be careful to take into account sensitive issues when choosing a means of eliciting an emotional reaction from their readers."