How do i stop my kitten from biting/scratching us? .?

we have a 6 mos. old kitten, and he can be agressive. when we try to pet him, or even if we are not doing anything, he'll try to bite/scratch us. when we pick him up, he chomps down on our hand and grasps us with his claws. he even attacks our legs when we walk by, trying to bite our feet or calf.