Is it important to choose a field of law before starting law school?

No. 

That having been said, some students who are in college seem to do better in their efforts to achieve high grades and are better able to focus on getting admitted to law school if they are driven by a passion for a particular branch of law.  One extreme example is Betty Anne Waters, who pursued a brief but stunningly effective career as an attorney. In 2001, Waters successfully overturned the murder conviction of her brother and enabled him to walk out of prison after 18.5 years. (Find out more about Ms. Waters at this link.)

Usually, students find that even if they enter law school with an idea of what they intend to do after becoming admitted to a State Bar, those ideas are significantly altered by financial circumstances, job availability, excitement about an area of law they knew little or nothing about - and for many other reasons.

Because most students across the country take the same classes during their first year, and many of the same classes during their second year, and because many state bar examiners test students on a wide variety of subjects, the opportunity to "major" (a term not used in law school) in a given topical area is rare.  (Although one may emphasize an area of expertise by taking advanced classes in the last half of the law school experience.)