Men:
(1) They should shave the entire head or trim the hair from all parts of the head.
(2) Shaving the head carries greater reward than trimming the hair, because the Messenger of Allah (peace and blessings be upon him) prayed three times for mercy and forgiveness for those who shaved their heads (رحم الله المحلقين). On the other hand, he made the said supplication only once for those who trimmed their hair (.... المقصرين).
(3) Trimming the hair from only part of the head will not suffice; rather, it is essential to trim the hair from the entire head.
There is no ruling for girls to shave their heads. They should only trim their hair.
There is no ruling for women to shave their heads. They should cut from the ends of one-fourth of the hair on their heads an amount equal to the upper joint of a finger (that is, slightly less than one inch). Girls should not cut more hair than this.
لَيْسَ عَلَى النِّسَاءِ حَلْقٌ إِنَّمَا عَلَى النِّسَاءِ التَّقْصِيرُ