I assume that from your question you need to choose an essay topic yourself based on this novel. With any dystopian fiction, an excellent essay topic would be to take one of the aspects of the speculative future that we are presented with and to use that as an essay topic, looking at the extent to which it is true now in our society or to what extent is it occurring now.
There are many such topics that you could select from this novel. An excellent one would be the blurring of the boundary line between reality and fantasy through media. Note the number of internet sites that Jimmy and Crake look at during their teenage years, including porn sites, sites of beheading and assisted suicides. Crake believes that they are all staged but it is clear that the effect of this media onslaught is to muddy the waters between the states of fantasy and fiction. To what extent is this happening now in our society?
Another topic you might want to think about is the potential for a disease to wipe out humanity. Is this a possible scenario? Could mankind we wiped out that easily?
Hope these ideas help!