The book explains that she has long, brown hair, pale face, almost translucent she's slender. and very clumsy
Extra: Copied from TwilightLexicon.com
Hair color: Brown
Eye color: Brown
Physical description: "Bella is very fair-skinned, with long, straight, dark brown hair and chocolate brown eyes. Her face is heart-shaped- wide forehead with a widow's peak, large, wide-spaced eyes, prominent cheekbones, a thin nose and a narrow jaw with a pointed chin. Her lips are a little out of proportion, a bit too full for her jaw line. Her eyebrows are darker than her hair and more straight than they are arched. She's slender but not at all muscular, and weighs about 115 pounds. She has stubby fingernails because she has a nervous habit of biting them."
You could go to wikipedia encyclopedia and type in Bella Swan, and you'll get a complete history about her.