Hi, it's been a while since I studied Frankenstein and when I did it was more concerned with Shelley's life in relation to the novel. Do a search on Frankenstein characterisation notes and that should bring some useful information up. Also, if you've got a copy of the book, there should be a preface in the front which should help. You could note how there are several references to Frankenstein's position as playing God. Apart from the obvious creator forming life theme of the book, there are referrals throughout the novel. The monster says in vol 2, chapter 2; 'I ought to be thy Adam; but I am rather the fallen angel'. Then in vol 2, chapter 7, he says; 'Like Adam, I was created apparently united by no link to any other being in existence; but his state...' and then a bit further down, 'God in pity made man beautiful and alluring, after his own image; but my form is a filthy type of yours...'. This is typical of the writing of the time by being preoccupied with the position of religion with emerging scientific discoveries. I'm not saying the bulk of your essay should be about this but it should definitely be discussed. Also, Frankenstein's constant feeling of shame and guilt (there are loads of quotes throughout) is typical of the Gothic novel.
I am mainly writing to say though that you will probably enjoy Of Mice and Men, The Catcher in the Rye and maybe The Dice Man or some Hubert Selby Jr if you're a worldly fifteen!
Good luck! x