How can I write a good essay describing what Mary Shelley's Frankenstein is suggesting about the consequences we face when we don't take...

I think that the first step you need to take in order to outline this essay is to decide what the plot of the novel says about this topic. In order make sense of that answer, you first need to decide which character you're going to focus on. I think that the natural choice is Doctor Victor Frankenstein, who creates the Creature but does not think through the consequences of that action, which of course results in the multiple tragedies that ensue. Clearly, the novel must take a very dim view of those who do not take responsibility for their actions. So, here is a brief example of a thesis and basic outline that might work for your purposes. This is only a jumping off point, and you will need to hone and refine this as you set about planning and writing your essay. Remember, writing is rewriting.

Thesis Statement: "Mary Shelly's novel, Frankenstein, illustrates the terrible consequences that ensue when a scientist, Doctor Frankenstein, decides to use his immense intellect to create a life from a scrapheap of corpses, without first considering how his creation will live in the world, and how the world will react to his creation." 

First Body Paragraph: Briefly describe what motivates Dr. Frankenstein to set about on his creation: pride, the hope for fame and glory. Remember that his motivations are mostly self-serving, and he does not heed anyone else's advice or warnings. Also explain how Victor feels that he has God-like powers, and focus on the passages that show him thinking about his own future, but not that of the creature, or the others who might interact with the creature.

Second Body Paragraph: Note what little precaution Victor takes when "building his creature." Everything he focuses on, when you think about it, is about "creating the monster," the mechanics, et cetera. He never stops to ponder how the creature will feel. He barely considers the creature human, and never stops long to think about the moral or psychological impacts this creation will have on the creature or the rest of the world. 

Third Body Paragraph: Here you should focus on how the creature destroys Victor's loved ones, and finally, how he destroys Victor. You should also focus on the emotional trauma that the creature suffers. In some ways, his is the cruelest punishment, to be utterly alone and left to wander the arctic tundra. Ultimately, you can show how everyone involved in the "experiment" lives to regret that it ever succeeded.