How can I compare two or more novels in the same essay?

It's important to allow yourself time to plan before you start writing. If you plan well, you'll find you can write the actual essay itself much faster and it will be clearer (less waffle!). If I have an hour to write an essay, I always use the first 15 minutes to plan, and the remaining 45 to write.
For me, either creating columns or a mind map really help when having to compare two novels/ poems/ etc. If you think of a main point from one novel that you want to talk about (i.e. a major theme), see if the theme appears in both novels and write about, for example, how each of the authors tackle that theme in their individual books. It's always good to say a little comment/ evaluate how well the authors approached the theme as well.