This is a great question! Both Heart of Darkness by Conrad and Things Fall Apart by Achebe are addressing the racist underpinnings of the colonization of Africa by European Christians. That comparison is where you need to start, but it is just the start.
First, go through each work and select quotes and passages that seem to you to not only be about Africa and Africans but that also bring in the attitudes and behaviors of the colonists—and of the authors. Each quote can be cited but also annotated, as both of these literary works are very famous and have a lot of scholarship available about them. I would just search each quote online and see what other scholars have written about that quote. Be sure to attribute all your quotes and rationales properly.
Next, Achebe is known for a notorious essay that was highly critical of Conrad's view of Africa and Africans. Here is where you can take your essay to the next level. After reading Achebe's article, which I have linked below, the next step would be to figure out whether you agree or disagree with Achebe's criticism of Conrad, and why.
I personally disagree with it, because Conrad in Heart of Darkness was actually highly critical of the colonization of Africa, but Achebe seems to have missed or misunderstood Conrad's uses of sarcasm, inference, symbolism, and other literary devices. Mine is a controversial take on Achebe's essay, but controversy makes for interesting essays, so don't be timid. You can use Achebe's references in his essay about Conrad to gather more quotes to use yourself. You could then criticize Achebe by pointing out places where it appears he misunderstood Conrad. Conversely, if you agree with Achebe's criticism, you can then cite your reasons why.
Once you have written your first draft, be sure to go back through it and organize your various ideas, each grouped together in their own space and in a logical progression. I wouldn't be surprised if the research for your essay does not draw you further into the scholarship on, and other works by, these two great authors. Good luck!