Kmcapello has given you some good advice to get you started. Once you have determined what traits and what summary information supports your thesis (the main idea of your essy - see attached link for more information), you need to also know how to integrate it into the text. Your teacher will likely want you to cite in either APA or MLA format. The basic premise is the same. You will want to note the author. For instance, Meyer describes Bella as "include quote from text here" (p. 22). Always provide a page reference, and cite the novel itself in your bibliography. This is important as it keeps you from being guilty of plagiarism and it provides you reader with a point of reference as to where to locate the quote in the novel! When summarizing, be sure to use only those elements that directly support your point. Be careful not to give a generalized "plot summary" when a single instance example will do.
A character analysis is not just a physical description; it uses text from a piece of literature in order present a complete picture of a character's personality and their role in the work. Since Bella Swan is the protagonist of Twilight, you will have a lot to work with.
I would suggest you first begin with a summary of Bella's character traits. Make a list of the ones you feel are most prominent or influence the action most. For instance, her clumsiness causes Edward to feel extremely protective. She is also pretty brave...though some may argue that it is foolishness rather than courage.
Also consider how Bella changes throughout the course of the novel. Does she grow up in any significant way, does she gain or lose any of her characteristics? How is she different from the beginning of the book?
Once you have a list of three or four important personality traits, return to the novel and find lines of text which describe those traits. Twilight is certainly long enough that you should have no problem finding lots of examples!