Vital records are one of the best sources for family tree information. These records are issued by state and local governments when a major life events happens (birth, marriage, death, etc.). In addition to describing the event, these records typically reveal other information too - like who the person's parents or next of kin, when and where they were born, etc. All these little pieces of information will help you piece together the larger picture ... that is, your family tree.
Due to privacy reasons, however, these records generally aren't readily available to the public. Your best source of vital records is to ask members in your own family for copies. I've been working the last couple of years to borrow every vital record (and photograph) I can get...