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!